Frei was born in Feldkirch, Austria, to a Turkish father and a Moroccan mother and was raised in Switzerland.
Career
Fulham
Frei made his first team début on 7 July 2011 in Fulham's Europa League first qualifying round, second leg match against NSÍ Runavík of the Faroe Islands, where he came on as a substitute in the 72nd minute replacing Andrew Johnson. Following his debut, Huw Jennings gave his thoughts about Frei debut to Fulham.com: Frei then went on to make his first start for Fulham, playing on the right wing in the Europa League second qualifying round, first leg match against Crusaders in Belfast on 14 July 2011, receiving a yellow card in the 62nd minute. He made his first start in English competition in a League Cup third round fixture away to Chelsea on 21 September 2011 and made his Premier League début on 10 December 2011, coming on a substitute for Mousa Dembélé in an away match against Swansea City. Fulham lost the match 2–0. Frei scored his first senior goal for Fulham in the Europa League 2–2 draw against Odense BK at Craven Cottage, side footing home from a Dembélé through ball. On 16 March 2012, Frei signed a contract extension with Fulham, lasting until the summer of 2015. A week later, Frei started in Fulham's 1-0 defeat at Old Trafford against Manchester United. He was replaced by Bryan Ruiz in the 67th minute. On 9 April 2012 in a 1–1 draw against Chelsea, Frei received the Man of the Match award from Sky Television for his performance. On 26 October 2012, Frei joined Cardiff City on loan until 2 December. His first appearance was against Burnley, coming on in the 83rd minute for Craig Noone. On 22 November, Frei was recalled to Fulham after 3 appearances.
Beşiktaş
Frei joined Turkish club Beşiktaş on 6 September 2013, signing a five-year deal. He made his league debut on 22 September 2013 in a 3-0 loss to Galatasaray, coming on in the 80th minute for Veli Kavlak. He scored his first league goal for Besiktas over a year later in a 3-2 loss to Kayseri Erciyesspor. He scored a brace in this game, one goal in the 70th minute, the other in the 85th, coming after he had come on for an injured Olcay Şahan in the 37th minute.
Birmingham City
On 20 January 2017, Frei returned to English football, signing for Birmingham City on a three-and-a-half-year contract. The fee was an initial €2.25 million, potentially rising to €3.5M. He made his debut in the EFL Championship the following day, as an 82nd-minute substitute in a 1–1 draw away to Blackburn Rovers, but never established himself in the team. He finished the season with 13 league appearances, only 3 of which were in the starting eleven, and was an unused substitute 16 times. He scored once, with a free kick away to Rotherham United on 14 April; Frei "stepped up and arrowed the ball straight and dipping beyond " to win the club's Goal of the Season award but not the match.
İstanbul Başakşehir
After just six months with Birmingham City, Frei returned to Turkey. He signed a four-year contract with İstanbul Başakşehir on 21 July 2017. He made his league debut for his new club on 19 August 2017 in a 3-1 loss to Kardemir Karabükspor. He spent the second half of the 2018–19 season on loan at Maccabi Haifa, and after returning to his parent club but playing little, joined Dutch Eredivisie club Emmen in January 2020 on loan with an option to purchase.
