At 16 years and six days Sinclair became Liverpool's youngest-ever player when coming on as substitute against West Bromwich Albion in the third round of the League Cup on 26 September 2012, beating the record easily set by Jack Robinson at Hull City in 2010. His first-team debut came only seven days after his first game for Rodolfo Borrell's Academy side in the NextGen Series against Inter Milan in Italy after impressing for the u-18 side. Sinclair joined Liverpool from West Brom's youth academy in the summer of 2011.
Sinclair played in just two Under-21 matches for Liverpool during the 2013/14 season, against Wolverhampton Wanderers in February and against Reading in March. He also made 18 appearances for the club's Under-18 team, scoring 13 times.
In March 2015 Sinclair, now 18 years old, moved on a youth loan to Championship club Wigan Athletic where he only made one appearance before returning to Liverpool. He made his Premiership debut for the club on 10 May 2015 when he came on against champions Chelsea. The following weekend he made another appearance as a substitute when coming on for Alberto Moreno towards the end of Liverpool's home defeat by Crystal Palace.
Sinclair's Liverpool future looked to be in doubt as 2015 drew to a close, with his contract coming to an end and Aidy Ward, who antagonised the club so much with the handling of Raheem Sterling's departure, becoming his agent. However he was handed a start in the 3rd round of the FA Cup way to Exeter City, where he struck his first goal for the club in the twelfth minute. Soon after this he announced his intention to leave at the end of the season, expressing a wish to play abroad, but instead agreed to sign for Watford on the expiry of his contract. He was an unused substitute in the opening game of the season and not named in the squad for the second. After one start and four substitute appearances in the Premier League he joined Championship-side Birmingham City on loan in January 2017. Things didnt work out as hoped there either, with all three of his starts coming in the first month of the spell and him failing to feature in the last eleven games.
During 2017-18 he was back at Watford, making just four substitute appearances totalling 27 minutes on the pitch. He joined League One side Sunderland on loan for 2018-19, but the agreement was cut short by Sunderland in January after he failed to pin down a regular place in the side. At the end of that January 2019 transfer window he joined Oxford United for the rest of the season. He was rejuvenated there, scoring four times in sixteen appearances to help the club pull away from the relegation zone to finish twelfth.