Ljajić began his career with FK Jošanica, and later Partizan, with whom he made his debut in the Serbian SuperLiga and UEFA Champions League.

In January 2010, he was set to join England's Manchester United,[2] but the club withdrew its option to sign him in December 2009.

He earned a total of 47 caps, scoring 9 goals,[6] and his final international was an October 2020 Nations League match against Hungary.

[9] Ljajić made an appearance for Partizan in the first leg of the second qualifying round of the 2008–09 UEFA Champions League on 29 July 2008 as a second-half substitute.

[12] However, United decided not to take up their option to sign Ljajić due to issues surrounding the club's application for a work permit.

After passing a medical, Ljajić signed a five-year contract,[16] with Partizan receiving a reported fee of €8 million for the player.

[4] Ljajić made his debut in Fiorentina's 2–2 draw away to Cagliari on 31 January 2010, coming on as an 82nd-minute substitute for Manuel Pasqual.

His reaction to this substitution was to sarcastically applaud the manager Delio Rossi, who grabbed and attempted to punch Ljajić.

[5] He chose the number 8 jersey, left vacant by Erik Lamela, which Ljajić also used with the Serbia national team.

[22] After Inter opted not to exercise their purchase option, Ljajić was sold to Torino on 18 July 2016 for €8.5 million, plus €500,000 in potential bonuses.

On 26 October 2010, it was announced by Serbia coach Vladimir Petrović that Ljajić would receive his first senior call-up against Bulgaria in a friendly match on 17 November 2010.

Ljajić was removed from his country's squad by head coach Siniša Mihajlović after refusing to sing the Serbian national anthem in a 2–0 friendly loss against Spain on 28 May 2012 for personal reasons.

[28] Ljajić previously signed a code of conduct established by Mihajlović that included, among other things, to sing the Serbian national anthem, "Bože pravde" (God of Justice).