Mehmet was released by manager Michael Brown, following the club's relegation in May 2017. Mehmet signed a one-year contract with Scottish Championship side Dundee United, after impressing after a trial in July 2017. He was restricted to cup appearances in the first part of the 2017–18 season, despite expecting a better opportunity when Csaba László replaced Ray McKinnon as manager in November 2017, although he explained that the change in management had little effect on him as he spent most of his time at Tannadice with goalkeeping coach Stuart Garden. Mehmet eventually staked his claim as first-team goalkeeper towards the end of that season, as United defeated Dunfermline Athletic in the play-off quarter-finals but had to be substituted for Harry Lewis just under an hour into the second leg, after he injured himself celebrating Scott McDonald's equalising goal. Manager Laszlo stated that "this was the funniest thing he had seen in football and was really stupid". Mehmet signed a new one-year contract with Dundee United in May 2018, however the torn cruciate ligament injury he sustained in the play-offs left him unable to feature during the first half of the 2018–19 season. On 29 March 2019, Mehmet signed for Queen of the South until the end of the 2018–19 season and Queens manager Gary Naysmith explained that he signed him after first-team goalkeeper Martin and third-choice goalkeeper Ryan Gibson both picked up injuries. He rejoined Dundee United on 5 July 2019 signing a two-year contract; sporting director Tony Asgha explained that he was allowed to leave the club in order to prove his fitness, and having done so he was re-signed to provide competition for first-choice goalkeeper Benjamin Siegrist. However Siegrist played all 28 league games in the 2019–20 season as United were declared as league champions with eight games left to play due to the COVID-19 pandemic in Scotland. Mehmet has been capped by Turkey at under-16, under-17, under-18, and under-19 level. He was selected by Abdullah Ercan as a squad member for the 2009 UEFA European Under-17 Championship in Germany and the 2009 FIFA U-17 World Cup in Nigeria.