On 1 July 2012. Zejnullahu signed to Union Berlin's under-19 team. In 2012, he made his debut with Union Berlin II. On 1 February 2013, he made his debut with the first team in a 2. Bundesliga match against Sandhausen coming on as a substitute in the 89th minute in place of Michael Parensen.[1]
On 19 May 2014, Zejnullahu received a call-up from Kosovo for a friendly match against Turkey and Senegal[9] and on 25 May 2014, he made his debut with Kosovo in a friendly match against Senegal replacing Albion Avdijaj in the 55th minute.