Taylor was born and raised in Bray, County Wicklow and is the daughter of an Irish mother Bridget (née Cranley) and British father Peter Taylor. Her father was born near Leeds and grew up in Birmingham and he first visited Bray to work with his father in the amusement arcades on Bray seafront. After meeting and marrying Bridget, Peter Taylor decided to settle in Ireland. In 1986, he was an Irish senior light heavyweight champion boxer. Originally an electrician by trade, Peter eventually became Katie’s full-time boxing coach. He also coached Adam Nolan who, like Katie, also represented Ireland at the 2012 Summer Olympics. Bridget also developed an interest in boxing and became one of the earliest female referees and judges in Ireland.

Taylor first began boxing in 1998, aged 11. Her father coached her and her two older brothers, Lee and Peter, at St Fergal’s Boxing Club, which operated out of a former boathouse in Bray. At 15, she fought the first officially sanctioned female boxing match in Ireland at the National Stadium and defeated Alanna Audley from Belfast.

Between 1999 and 2005 Taylor attended St. Kilian’s Community School in Bray. Her three older siblings all attended the same school. As well as boxing and playing association football, as a schoolgirl Taylor also played ladies’ Gaelic football and camogie with her local GAA clubs, Bray Emmets and Fergal Ógs. She was also a member of Bray Runners, a local athletics club. Several American colleges reportedly offered her sports scholarships while she was still a pupil at St Killian’s. However, she opted instead to attend UCD. Although UCD is well known for sports scholarships, Taylor qualified via her Leaving Cert results. However, as Taylor’s sporting career began to take off, she chose not to complete her studies at UCD.

In 2002, at the age of 15, she appeared on RTÉ’s Sport Stream and discussed her dream of one day appearing at the Olympics.

As a schoolgirl, Taylor played association football for St Fergal’s and Newtown Juniors in the Wicklow and District Schoolboys League. Taylor played in teams with boys. She also represented the league in the Kennedy Cup, an inter–league competition, and won the league’s Player of the Year award. At senior club level, Taylor played in the Dublin Women’s Soccer League for Lourdes Celtic, St James’s Gate, St Catherine’s and Peamount United. Taylor played for St James’s Gate in the 2003 DWSL Premier Cup final against Dundalk City. Gate lost 3–2 after extra time. Taylor also played for Peamount United in the 2005 FAI Women’s Cup final. However Peamount lost 1–0 to Dundalk City. In 2009 Taylor was a member of the St Catherine’s squad that reached the FAI Women’s Cup final. However, her boxing commitments saw her miss out on the final itself. In 2010, together with Nicola Sinnott, Áine O’Gorman, Sara Lawlor and Louise Quinn, Taylor was a member of the Peamount United squad that won a treble, winning the Dublin Women’s Soccer League, the DWSL Premier Cup and the FAI Women’s Cup.

Taylor represented the Republic of Ireland women’s national football team at under–17, under–19 and senior levels. She was 14 when she played for the under–17s and 15 when she started playing for the under–19s. According to Taylor, her boxing training helped her football career because it made her physically strong enough to bridge the age gaps. On 25 September 2003, Taylor scored four goals for the Republic of Ireland under–19s in a 2004 UEFA Women’s Under-19 Championship qualifier against Macedonia. Between 2006 and 2009, Taylor made 11 appearances and scored 2 goals for the senior team. She made her senior international debut on 22 April 2006 against Switzerland in a 2007 FIFA Women’s World Cup qualifier at Richmond Park. The Republic of Ireland won 2–0. In 2006 Taylor also played for the Republic of Ireland in a prestige away friendly against the United States. Taylor scored her first senior international goal on 1 April 2007 in a UEFA Women’s Euro 2009 qualifier against Hungary at Tolka Park. Taylor’s goal helped the Republic of Ireland win 2–1. She scored her second goal in the same qualifying campaign in a 4–1 defeat against in Italy on 16 February 2008. She was also sent-off in the same game. She made her final international appearance on 24 September 2009 when she came on as substitute in a 2011 FIFA Women’s World Cup qualifier against Kazakhstan.

Later in 2005, at the World Amateur Championships in Podolsk, Russia, Taylor advanced to the quarter-finals in the 60 kg weight class. Here she lost against Kang Kum-Hui, who remains the only boxer to have defeated Taylor at the World Championships.

At the 2006 European Amateur Championships in Warsaw, Poland, Taylor won her second successive gold medal by stopping reigning world champion Tatiana Chalaya of Russia, also collecting the tournament’s Best Boxer award.

At the 2006 World Women’s Boxing Championship, contested in New Delhi, India, Taylor became Ireland’s first World Champion, defeating Chalaya again in the semi-final and then Erica Farias of Argentina in the 60 kg final.

In 2007, she won her third successive European Championship title in Denmark.

2008 saw Taylor win her first European Union gold medal, contested in August in Liverpool, England. Here she defeated Cindy Orain of France.

On 21 March 2009 at The Dublin O2, Taylor won a 27–3 win over three-time Pan-American champion Caroline Barry of the United States on the undercard of a pro WBA super bantamweight world title fight between Bernard Dunne of Ireland and Ricardo Cordoba of Panama. Speaking after the fight, Taylor, who had stopped Barry in the final of the 2006 World Championships in New Delhi, said she was stunned by the welcome she received from Irish boxing fans. She said: “I couldn’t believe the reception I got – it was an amazing experience for me. I knew it was going to be a tough fight and well done to her for never backing off.”

Taylor defended her European Union title in 2009. She beat home favourite, Bulgaria’s Denitsa Elisayeva, in the July tournament hosted in Pazardzhik.

She was a coach on RTÉ’s Charity Lords of the Ring in 2009.

On 18 September 2010, Taylor went on to claim her third successive World title at the 2010 AIBA Women’s World Boxing Championship, in Barbados. In the 60 kg weight class, she again defeated China’s Cheng Dong in the final match. This was Taylor’s 100th career win.

Taylor won the gold medal at the EU Women Boxing Championships in Katowice, Poland in 2011.

Taylor appeared in a Lucozade Sport advertisement in 2011 alongside English rapper Tinie Tempah and American musician Travis Barker.

On 19 May 2012, Taylor won her fourth successive World title at the 2012 AIBA Women’s World Boxing Championship, in Qinhuangdao China. In the 60 kg weight class, she defeated Russian southpaw Sofya Ochigava.

In the semi-final on 8 August 2012, she proved far too good for Tajikistan’s Mavzuna Chorieva and won in a 17–9 victory (R1: 3–1, R2: 4–2, R3: 6–3, R4: 4–3), booking her place in the final and guaranteeing her, at least, an Olympic silver medal.

Taylor defeated Russia’s Sofya Ochigava in the final bout by 10–8 (R1: 2–2, R2: 1–2, R3: 4–1, R4: 3–3) on 9 August 2012, winning an Olympic gold medal, and becoming the first ever Olympic female lightweight champion.

On 24 November 2014, Taylor won her fifth straight lightweight title in South Korea at the 2014 AIBA Women’s World Boxing Championships, defeating Yana Allekseevna of Azerbaijan. The final scoreline was 40–36, 39–37, and 39–37 in her favour.

On 27 June 2015, Taylor won the lightweight title in Azerbaijan at the inaugural European Games, defeating Estelle Mosely of France. The final scoreline was 40–36, 40–36, and 39–37 in her favour.

On 24 May 2016, Taylor qualified for the 2016 Summer Olympics after defeating Victoria Torres in the quarter-finals of the lightweight division at the 2016 AIBA Women’s World Boxing Championships. Two days later, Taylor lost to Estelle Mossely in the semi-finals which ended her quest for a sixth World title in a row.

On 15 August 2016, Taylor lost 2–1 in the quarter-finals to Mira Potkonen of Finland and did not advance.

Taylor made her professional debut on 26 November 2016, scoring a third-round technical knockout (TKO) at the Wembley Arena in London.

In her second professional bout she faced Viviane Obenaufon on 10 December 2016 at the Manchester Arena, Manchester. The fight was televised live on Sky Sports Box Office as part of the undercard for the Anthony Joshua vs. Éric Molina heavyweight world title fight. In a bout which saw Taylor score a knockdown in the second round after Obenauf appeared to slip, and the latter suffer a cut to her left eye following an apparent clash of heads, Taylor won by a shutout six-round points decision (PTS) with the referee scoring the bout 60–53.

She scored two more wins in March 2017—against Monica Gentili via fifth-round TKO and Milena Koliva via PTS over eight rounds—before facing Nina Meinke for the vacant WBA International female lightweight title on 29 April 2017 at the Wembley Stadium. The bout served as a final eliminator for the WBA female lightweight title and was televised live on Sky Sports Box Office as part of the undercard for the Anthony Joshua vs. Wladimir Klitschko world title fight. Taylor controlled the contest from the opening bell with her speed and combination punches. Meinke suffered a cut in the fifth round from an accidental clash of heads. With a badly swollen right eye, and after being on the receiving end of a flurry of punches, referee Howard Foster called a halt to the contest in the seventh round to save Meinke from further punishment, handing Taylor a TKO victory.

Following a third-round stoppage victory via corner retirement (RTD) against Jasmine Clarkson in July, Taylor faced former two-weight world champion Anahí Ester Sánchez for the vacant WBA female lightweight title on 28 October 2017 at the Principality Stadium in Cardiff, Wales. Sánchez was set to defend the title on the night, however, she failed to make weight for the fight and was subsequently stripped by the WBA, with the title on the line for Taylor only. The bout was televised live on Sky Sports Box Office as part of the undercard for Anthony Joshua vs. Carlos Takam. After controlling the first round with her speed and power, Taylor dropped the former champion to the canvas with a body shot in the second. Sánchez was hurt again in the next round after a right hand from Taylor caused her to stagger. Sánchez had some success in the fifth, landing right hands to force Taylor onto the back foot. Taylor regained control for the rest of the fight to secure a wide unanimous decision (UD) victory, with all three judges scoring the bout 99–90 to hand Taylor her first world title.

Her next fight was a unification bout against reigning IBF female lightweight champion Victoria Bustos on 28 April 2018 at the Barclays Center in New York City. Taylor controlled the majority of the fight with her speed from a distance, only encountering difficulty in rounds seven and eight after engaging Bustos at close range. Taylor secured the win with a wide UD to add the IBF title to her growing collection, with two judges scoring the bout 99–91 and the third scoring it 98–92.

She made three successful defences of the her titles in 2018; defeating Kimberly Connor via third-round TKO in July; former WBO female featherweight champion Cindy Serrano by UD in October; and former WBC female super-featherweight champion Eva Wahlström in December.

In December 2018, Irish journalist Evanne Ní Chuilinn stated that Taylor accused Delfine Persoon of turning down a “life-changing” amount of money to fight her. Later Persoon claimed to have offered $100,000 to Taylor for a fight. Taylor’s manager stated that Taylor wouldn’t even get out of bed for this sum. On 15 April 2019, Taylor and Persoon agreed to a lightweight title unification bout, to take place on 1 June at Madison Square Garden as undercard for the Anthony Joshua vs. Andy Ruiz Jr. heavyweight title fight. In addition to all four sanctioning body’s titles being on the line, The Ring magazine belt was also at stake.

In 2018, the documentary Katie was released, chronicling her early life to her first title shot.

For her next fight Taylor elected to move up in weight, facing WBO female junior-welterweight champion Christina Linardatou on 2 November 2019 at the Manchester Arena. The champion started the fight as the aggressor, forcing Taylor to fight off the back foot. The Irish challenger found success at the end of the round with two solid right hands as the bell rang. Taylor started to find her rhythm in the second round, scoring punches while using her fast footwork to make her opponent miss. The third round saw more of the same with Taylor staying behind the jab and landing left hooks. Linardatou found success in the fourth and fifth rounds, landing multiple right hands and leaving Taylor with swelling above her right eye. Taylor began regaining control in the sixth, using her footwork and landing her signature rapid combinations. Round eight saw Taylor plant her feet and trade punches, landing a combination of power punches before taking a solid right hand from the champion that stopped Taylor in her tracks. After critical comments from her corner for taking too many punches in the eighth round, Taylor got back to her boxing for the next two rounds, coasting to a UD victory with the judges scorecards reading 97–93, 97–93, and 96–94. With the win Taylor became only the third boxer from Ireland, after Steve Collins and Carl Frampton, to capture world titles in two weight divisions.

Taylor was scheduled to defend her lightweight titles against seven-division world champion and current WBO and WBC interim female featherweight champion Amanda Serrano on 2 May 2020 at the Manchester Arena. However, due to the COVID-19 pandemic the bout was postponed and rescheduled to 4 July, with the venue changing to the Matchroom Sport headquarters in Brentwood, Essex. The bout was once again postponed, with a new proposed date of 22 August. After negotiations began to break down, Taylor’s promoter, Eddie Hearn, revealed he was in talks with Delfine Persoon’s team to secure a rematch for the 22 August date. On 9 July, it was confirmed that the Taylor vs. Persoon rematch will take place on 22 August at the Matchroom Sport headquarters in Brentwood, Essex, airing live on Sky Sports Box Office as part of the undercard for Dillian Whyte vs. Alexander Povetkin.