Lončar made his professional debut with hometown team Sutjeska. In September 2016, he joined Serbian team FK Rad on the last day of the summer transfer window.[3] After six months at Rad, Lončar didn't play a game and returned to Sutjeska.[4] After two more seasons at Sutjeska, he was recognized as the best player of the Montenegrin First League in 2018.[5]
Alavés
Loan to Istra 1961
On 20 January 2019, it was announced that Lončar signed a contract with Spanish club Alavés, lasting until the summer of 2021.[6] Upon signing, he was immediately sent on a one-season loan to Croatian club Istra 1961, whose ownership was bought by Alavés in the summer of 2018.[6] On 29 September 2019, he scored his first goal for Istra in a 1–1 tie with Hajduk Split.[7]
Debrecen
On 10 January 2023, Lončar moved to Debrecen in Hungary.[8] On 31 July 2023, he scored two goals against Mezőkövesdi SE on the first match day of the 2023–24 Nemzeti Bajnokság I season. The final result was a 3–1 victory for Debrecen.[9]