Born in Södertälje, Sweden, Ishak started his professional career at Assyriska FF, where he was included in the first team in 2010. In December 2011, Ishak was signed by German Bundesliga club 1. FC Köln after a successful trial. He made his debut for the side on 21 January 2012 in a 0-1 away defeat at VfL Wolfsburg, where he came on as a substitute in the 85th minute for Martin Lanig. After being used mostly as a substitute during his first month in Köln, Ishak made his first start for the club on 18 March following a suspension for regular striker Lukas Podolski, playing the entire match in a 1-4 away loss at Hannover 96. Ishak would make 11 appearances for 1. FC Köln during his first season. In February 2013, Ishak signed a six-month loan deal with Swiss club FC St. Gallen, who had recently been promoted to the Swiss Super League, in order to play more. He became a regular starter for the club, before suffering a mandibular fracture in a match for the Sweden under-21 team. Ishak made his return in late-April as a substitute in a match against FC Zürich, before becoming a full-time starter again the following week. He made a total of 13 league appearances for St. Gallen, in which he scored three goals. Ishak joined Italian Serie A club Parma on a four-year contract on 5 August 2013. He was loaned out to Crotone, competing in the Serie B, following his arrival. He made his debut for Crotone on 24 August 2013 in a match against Siena. On 24 September, he scored his first goal for the side in a 3-1 home win over Modena. In August 2014, a year after joining Parma, Ishak moved to Danish Superliga club Randers FC, signing a three-year contract. He would play for the club for two-and-a-half years, scoring 31 goals in 71 league appearances. After a highly successful stay at Randers, Ishak joined German 2. Bundesliga club 1. FC Nürnberg in January 2017, six months before his contract expired. He scored Nürnberg's 1000th goal in the 2. Bundesliga on 25 November 2017 in a match against Eintracht Braunschweig. At the end of the 2017–18 season, he reached promotion to the Bundesliga with the club. Ishak scored his first goal in the Bundesliga on 1 September 2018, an important equalizer in the away game against 1. FSV Mainz 05, which ended 1-1.
