*Club domestic league appearances and goals, correct as of 20:03, 21 December 2024 (UTC) ‡ National team caps and goals, correct as of 22:47, 18 November 2024 (UTC)
In the summer of 2014, Mahmutović signed his first professional senior contract with CS Fola Esch.[5][6]
On 19 May 2017, it was announced that he had joined Championship club Middlesbrough for an undisclosed fee.[7] Mahmutović made his debut for the club on 14 August 2018, in an EFL Cup tie with Notts County, scoring a headed goal in the 3–3 draw.[8] In doing so, he became the first Luxembourger in history to score a professional goal in English football.[9]
On 30 August 2018, Mahmutović joined League Two club Yeovil Town on loan until the end of the season.[10] His time at Yeovil Town was cut short on 31 December 2018, with Mahmutović returning early to Middlesbrough upon the opening of the January transfer window after only playing a limited role at the League Two side.[11]
On 1 July 2019, Mahmutović joined MVV Maastricht of the Dutch Eerste Divisie, going on to appear 16 times for the club across the season.[12]
Middlesbrough announced that Mahmutović had been released by the club on 28 June 2020.[13]
On 31 March 2022, Mahmutović signed a contract with Bulgarian club CSKA Sofia.[14] He left the team in June 2024, upon the expiration of his contract.[15]
On 29 July 2024, Mahmutović moved to NAC Breda in the Netherlands, signing a contract for two seasons with an option for third.[16]