Grealish’s younger brother, Keelan, died of sudden infant death syndrome in April 2000 at the age of nine months.

He is of Irish descent, through his maternal grandfather from County Dublin, his paternal grandfather from Gort, County Galway, and his paternal grandmother from Sneem, County Kerry. Influenced by his Irish heritage, Grealish played gaelic football for John Mitchel’s Hurling and Camogie Club of Warwickshire GAA between the ages of 10 and 14. He competed against former Aston Villa Ladies and current Birmingham City Ladies defender Aoife Mannion, a school classmate of his, at Gaelic football. On 4 August 2009, Grealish scored a point representing Warwickshire GAA at Croke Park during half-time of the 2009 All-Ireland Senior Football Championship quarter-final between Dublin and Kerry.

While playing in Irish youth teams, England were known to have been pursuing him, even naming him in their under-17 team in 2011 at the age of 15 – an invitation he declined. After being left out the Republic of Ireland under-21 team for three qualifiers in October 2012, the English FA made an approach for him to switch. Republic of Ireland under-21 manager Noel King said in May 2013 that the 17-year-old was pondering a switch to England so he was not considered for a friendly against Denmark, although King later stepped in to assure Grealish and his family that he was a part of his plans.

Grealish, a lifelong Aston Villa fan, joined the club as a six-year-old. At the age of 16, he was named as an unused substitute in a 4–2 home Premier League defeat against Chelsea on 31 March 2012. Grealish was part of the club’s under-19 team that won the 2012–13 NextGen Series, scoring in a 3–1 extra-time win over Sporting CP in the semi-final.

On 13 September 2013, Grealish joined League One club Notts County on a youth loan until 13 January 2014. He made his professional debut the following day, coming on as a 59th-minute substitute for David Bell in a 3–1 away defeat to Milton Keynes Dons. On 7 December, he scored his first career goal, beating three defenders to score the last goal in a 3–1 win over Gillingham at Meadow Lane, and followed this a week later by opening a 4–0 victory at Colchester United. Grealish extended his loan with Notts County on 17 January 2014 until the end of the season. He ended his loan with five goals and seven assists in 38 appearances.

Grealish made his under-21 debut for the Republic of Ireland as a late substitute against the Faroe Islands in August 2013. In 2013, Grealish reaffirmed his desire to continue representing Ireland.

In May 2014, Grealish played in the Hong Kong Soccer Sevens, he finished as top scorer with six goals as Villa won the Shield title. With his contract due to expire in the summer of 2015, he was offered a new four-year deal by the club in September 2014. On 14 October, Grealish signed a new four-year contract with Aston Villa.

In August 2014, Grealish was again named to the Republic of Ireland under-21 squad. It was initially reported that he would decline the call up to the under-21s due to being undecided over his international future, however Grealish did turn out for the Republic of Ireland in a 2–0 loss against Germany. It later emerged that Grealish had actually declined a call-up to the senior Irish team after talks with Martin O’Neill. In October 2014, Grealish pulled out of a Republic of Ireland under-21 squad for a game against Norway to play in a behind closed doors friendly for his club Aston Villa and England under-21 manager Gareth Southgate confirmed that the Football Association were monitoring the player’s situation. Reports emerged on 17 October that Grealish had declared for Ireland and would make his senior debut the next month but this was denied by the player. Grealish was awarded the Under-21 player of the year by the Football Association of Ireland in March 2015 where he announced that he had taken a break from youth internationals over the past year to focus on breaking into the Aston Villa first team and that he expected to be back playing for Ireland in the near future.

As a consequence of his nimble movement, Grealish has notably been on the receiving end of heavier, more physical challenges from opposing players. Shaun Derry, his manager at Notts County, highlighted this, following fixtures against Sheffield United and Stevenage in early 2014 and called for more official protection. In a 2014 match for Aston Villa against Hull City at Villa Park, a number of fouls committed against Grealish resulted in three Hull players receiving yellow cards within just a 15-minute period.

Grealish made his first start in an FA Cup third round tie on 4 January 2015 against Blackpool at Villa Park, which his team won 1–0. He played 75 minutes before being substituted for Andreas Weimann. On 7 March, in the sixth round, a 2–0 home win over West Bromwich Albion, he replaced Charles N’Zogbia after 74 minutes, and was sent off for a second booking for diving in added time. On 7 April, Grealish started his first match for Aston Villa in the Premier League, a 3–3 home draw against Queens Park Rangers in which his performance was highly praised. In the FA Cup semi-final against Liverpool at Wembley Stadium, Grealish played a part in both of Villa’s goals including assisting Fabian Delph’s winner, as they came from behind to advance to the final. On 30 May, Grealish played the entirety of the 2015 FA Cup Final at Wembley Stadium, as Villa lost 4–0 to Arsenal.

In April 2015, Grealish was warned by Aston Villa manager Tim Sherwood after The Sun published images showing him allegedly inhaling nitrous oxide for recreational purposes. Sherwood said that “We can’t condone that behaviour. He is now in a responsible position as a professional footballer, he’s got to make sure it won’t happen again, he’s assured me it won’t. But, as I said about Raheem last week, he’s a young man, he was even younger a year earlier when the picture was taken.”

Grealish scored his first goal for Villa on 13 September 2015, a 20-yard shot to open the scoring away to Leicester City; however, his team lost 3–2.

In May 2015, O’Neill confirmed that Grealish had turned down another call-up to the Irish senior squad, this time for a friendly against England and a European Championship qualifier against Scotland. England manager Roy Hodgson disclosed that although he had been in contact with Grealish, he had chosen not to include him in their squad to face Ireland in case of a backlash. In August 2015, Hodgson met with Grealish to discuss his future.

On 28 September 2015, Grealish confirmed that he had decided to represent England at international level. He made his debut for England under-21s on 19 May 2016 as a 72nd-minute substitute for Ruben Loftus-Cheek in a 1–0 win over Portugal at the Toulon Tournament. On his first start four days later he scored twice in the first half of a 7–1 win over Guinea. England went on to win the tournament for the first time since 1994. Grealish was named in the squad for England under-21 at the 2017 UEFA European Under-21 Championship. From 2016 to 2017, he made seven appearances for the under-21s, scoring two goals.

Aston Villa manager Tim Sherwood said in May 2015 that Grealish was learning from his friendship with his midfield partner, the experienced Joe Cole. Sherwood considered Cole a role model for Grealish’s private life, as he “didn’t read too much about Joe being on the front pages”.

On 7 January 2016, Leeds United head coach Steve Evans said that Aston Villa had rejected an enquiry to take Grealish on loan. Villa finished the season in last place, ending their status as Premier League ever-presents. Grealish played 16 matches, all defeats, breaking a record for worst season previously held by Sunderland’s Sean Thornton, who lost in all 11 of his appearances in 2002–03.

In September 2016, Aston Villa opened an internal disciplinary investigation after reports that Grealish was at a party at a Birmingham hotel which had to be shut down by the police in the early morning. In response, owner Tony Xia wrote on Twitter that Grealish had to focus on and off the pitch, and associate with the right people. In October, Grealish was suspended for three matches after accepting a charge of violent conduct following a stamp on Conor Coady in Villa’s 1–1 draw with Wolverhampton Wanderers.

On 10 March 2019, Grealish was assaulted by a pitch invader during the derby match away to Birmingham City. Later in the second half, Grealish scored to give Aston Villa a 1–0 victory. The same day, a 27-year-old man from Rubery was arrested. He appeared on 11 March at Birmingham Magistrates’ Court charged with encroachment on to the pitch and assault. He pleaded guilty to the offences and was sent to prison for 14 weeks.

Grealish’s first goal of the 2019–20 season came in the second round of the 2019–20 EFL Cup against Crewe Alexandra on 27 August 2019. His first Premier League goal of the season came on 5 October, netting his side’s third goal in a 5–1 away win over Norwich City. The resulted lifted the club out of the bottom three and leapfrogged their opponents in the Premier League table.

In March 2020, the Premier League was suspended midway through Aston Villa’s return season, due to the COVID-19 pandemic in the United Kingdom. During that enforced break, it was revealed that Grealish had violated government guidance to stay home. He accepted that his actions were “wrong and entirely unnecessary” and was fined by the club.

On 15 September 2020, Grealish signed a new five-year contract with Aston Villa until 2025.

On 31 August 2020, for the first time, Grealish was called up to the senior England squad for the UEFA Nations League fixtures against Iceland and Denmark. On 8 September, he made his senior debut as a 76th-minute substitute in a 0–0 draw against Denmark.