The winger who can feature both on the left and the right side arrived from Newcastle United in 2015. Longstaff has become a firm fixture in Liverpool's U-23 side and was an unused substitute when the Reds knocked MK Dons out of the Carabao Cup in the third round on 25 September 2019. Longstaff made his debut for Liverpool in the quarter-finals of the Carabao Cup against Aston Villa on 17 December 2019.

During 2020/21 he was a regular for the Under 23's in Premier League 2, playing across the front three. For 2021/22 he was loaned to Scottish League One side Queens Park, scoring four times in 28 appearances as they were promoted to the Scottish Championship via the play offs but his contract at Liverpool was not renewed. At the beginning of August 2022 he agreed a one year deal with Cove Rangers, who like Queens Park had been promoted from Scottish League One. Cove were relegated from the Championship but Longstaff remained at that level for 2023/24 when he was offered a two year contract with Inverness Caledonian Thistle. He was a regular in midfield throughout the campaign, which ended in relegation to the Scottish third tier.

During 2024/25 Inverness CT were hit with a 15 points deduction but managed to avoid relegation, with Longstaff scoring twice in 25 league appearances. As soon as the season ended he committed to the club for another two years, saying it had been his most enjoyable campaign for a long time and praised the coaching staff for using him in his preferred right wing position, encouraging him to beat his marker and get crosses into the box.