Upon returning from Qatar, Liverpool recorded a 4–0 league win over second-placed Leicester City during a match in which Alexander-Arnold played a hand in all four goals by scoring one, assisting two, and winning a penalty which was converted by Milner. He was later rewarded for his form with the Premier League Player of the Month award, becoming the first full-back to claim the honour since Micah Richards in 2007. Further recognition followed at the turn of the year, when he was named in the 2019 UEFA Team of the Year. Alexander-Arnold then played a starring role in Liverpool’s 2–1 win over Wolves as the club extended their unbeaten run in the league to 40 matches. During the encounter, he assisted captain Jordan Henderson for the opening goal and in the process became the first defender in the competition’s history to reach double figures for assists in multiple campaigns.

Having previously taken part in Liverpool’s pre-season tour of the United States, Alexander-Arnold made his professional debut on 25 October 2016, starting in a 2–1 victory over Tottenham Hotspur in the fourth round of the EFL Cup. He was booked in the first half for a foul on Ben Davies, which he later revealed could have ended in him being sent off, before being substituted for first-choice right-back Nathaniel Clyne in the 68th minute. Despite the indiscretion, his performance was lauded by Gerrard who once again backed him “to become a top professional”, and also earned a spot in the EFL Cup Team of the Round alongside teammate Daniel Sturridge.

Alexander-Arnold’s breakthrough into the first team was rewarded with a new long-term contract in November and he started in Liverpool’s next EFL Cup match later that month against Leeds United. He was named Man of the Match for his performance after registering his first assist for the club by setting up striker Divock Origi for the opening goal of a 2–0 win for Liverpool. He then made his Premier League debut on 14 December, coming on as a late substitute in a 3–0 win over Middlesbrough, and made his first league start in a 1–1 draw with Manchester United on 15 January 2017. In May, having made 12 appearances across all competitions, he was named Liverpool’s Young Player of the Season and was also nominated for the Premier League 2 Player of the Season award.

In the buildup to the 2017–18 season, regular right-back Clyne suffered a serious back injury which afforded Alexander-Arnold the opportunity to rotate with Joe Gomez during the early stages of the campaign. On 15 August 2017, he scored his first goal for the club when he struck home from a free kick in a 2–1 first leg Champions League play-off round win over Bundesliga team Hoffenheim. In doing so, he became the third youngest player to score on his European debut for Liverpool, after Michael Owen and David Fairclough. During the group stages of the competition, Alexander-Arnold scored again in a 7–0 Champions League win over Maribor on 17 October, a result which was the joint-largest ever away win in the competition, and largest away win by an English club. He then scored his first Premier League goal for Liverpool on Boxing Day, netting in a 5–0 win over Swansea City at Anfield.

In March 2018, while part of the U21 team, Alexander-Arnold was invited to train with the senior national team for the first time ahead of their friendly matches against Italy and the Netherlands. He received his first call-up in May 2018 when he was named in Gareth Southgate’s squad for the 2018 FIFA World Cup. His debut followed on 7 June 2018 when he started in a 2–0 pre-tournament friendly win over Costa Rica at Elland Road. Prior to the match, he was handed his match jersey by Prince William, Duke of Cambridge. Alexander-Arnold then made his debut in the competition on 28 June, starting in a 1–0 group-stage defeat to Belgium, after both sides had already confirmed their progression to the knockout stages. In doing so, he became only the fourth teenager to start a match for England at a World Cup. It remained, however, his only appearance as Kieran Trippier was preferred at right-back and featured throughout as England were defeated by Croatia in the semi-finals, and then again by Belgium in the third place play-off.

On 15 November 2018, during a friendly international, held in honour of Wayne Rooney, Alexander-Arnold scored his first senior international goal as England recorded a 3–0 win over the United States at Wembley Stadium. In doing so, and at the age of 20 years and 39 days, he became the youngest Liverpool player since Michael Owen in 1999 to score for the national team. In June 2019, he was part of the England squad which ended third at the inaugural UEFA Nations League and played the full match in a penalty shoot-out win over Switzerland in the third-place play-off.

He is also an avid chess player, having been introduced to the sport by his father as a youngster, and in 2018 he played an invitational match against world champion Magnus Carlsen. The match, which was played as part of a campaign to promote the sport, ended in defeat for Alexander-Arnold after seventeen moves; eight more than technology entrepreneur Bill Gates managed when he faced Carlsen in 2014. Alexander-Arnold later explained how chess was more than just a pastime and had helped him in his football career by teaching him to think “two or three moves ahead of the opposition”.

During the early stages of the following season, he made his 50th appearance for Liverpool when he started in a 2–1 victory over Tottenham; the same opposition against whom he had made his debut almost two years prior. In October, he was one of 10 players nominated for the inaugural Kopa Trophy, an award presented by France Football to the best young player under the age of 21, and ultimately ended sixth in the voting polls, having received votes from Owen, Denis Law and Pavel Nedvěd. He continued to enhance his burgeoning reputation in the months thereafter and was consistently listed in reports by the CIES as the world’s most valuable full-back from a transfer value perspective. Alexander-Arnold elevated his game further following the turn of the year and, on 27 February 2019, became the youngest player, aged 20 years and 143 days, to record three assists in a single Premier League match when he set up Sadio Mané twice and Virgil van Dijk once in a 5–0 win over Watford. Less than two months later, he became the fifth-youngest player to reach 50 Premier League appearances for the club, behind Owen, Raheem Sterling, Robbie Fowler and Gerrard, when he started in a 3–1 win over Southampton. His domestic form was rewarded later in April when he was nominated for the PFA Young Player of the Year award, although the award was won by Manchester City’s Sterling, and he was later named in the PFA Team of the Year alongside Liverpool teammates van Dijk, Mané and Andrew Robertson.

Outside of football, Alexander-Arnold volunteers as an ambassador for the Liverpool-based charity An Hour for Others, which seeks to provide underprivileged members of the community with anything from food hampers and toys to cooking and science lessons. He has supported the charity since being introduced to it by his mother in his mid-teens. During his time with Liverpool’s academy, he and teammate Kris Owens pledged to support the initiative if either of them made it as professional footballers. His philanthropy also expands beyond the charity and in March 2019, after signing a new boot sponsorship with Under Armour (the second most lucrative boot deal in England behind Harry Kane’s) he initiated plans to purchase plots in Liverpool and use the funds to construct new pitches for the community.

In 2020, he and fellow Liverpool full-back Andrew Robertson starred in their own digital series on IGTV called ‘Wingmen’.