He joined the first team in 2014 under coach Nenad Lalatović, and made his senior debut for Red Star in a friendly match against Udinese on 15 November 2014.
[3] Later that month, Mijailović remained on the bench as an unused substitution in the 14th round of the 2014–15 Serbian SuperLiga season against OFK Beograd.
[4] During the winter break, Mijailović was loaned to the Serbian First League side Kolubara, but he remained in the youth team throughout 2015.
[8] After fully recovering from an injury he picked up in early 2016,[9] Mijailović was loaned to Bežanija,[10] where he made nine appearances and scored one goal until the end of the 2015–16 season.
[12] Although he spent the mid-season training with Red Star, head coach Miodrag Božović decided to leave him with Bežanija until the end of season.