Fenger started playing in the youth system at FC Roskilde,[2] before signing with OB on 8 August 2018, after scoring 26 goals in a season in the U17 league for Roskilde.[3] He initially became part of the under-19 team of OB.[4]
He made his professional debut on 1 June 2020 in a 1–0 away loss to AGF, coming on as a substitute in the 47th minute for Mart Lieder.[5] His first goal for the club came in the play-offs for European football, scoring in a 2–1 away loss to Randers FC on 11 July after making a start.[6] In the return game, he secured advancement for OB by scoring a brace as the team won 2–0 at home.[7] Fenger finished his first season in the first team with 10 appearances, in which he scored four goals – all in the play-offs for European football, which OB eventually lost out after a 1–0 loss to AGF.[8]
Fenger struggled to find playing time the following season, as he was sidelined with a knee injury which he suffered in a loss to Brøndby IF on 8 November 2020.[9] He made his comeback on 9 April 2021 in a 0–1 home loss to Vejle Boldklub as a late substitute.[10][9]
On 23 March 2023, Fenger was sent on loan to Allsvenskan club Mjällby AIF until 31 July 2023.[11][12] On 27 July 2023 the club confirmed, that the loan spell had been extended until the end of 2023.[13]