Kačar played forVojvodina's youth academy before being promoted to the first team. In January 2008, he moved toGermany to play forHertha BSC for a fee of €3 million, which made him the most expensive player that Vojvodina ever sold.
On 21 August 2010, Kačar debuted forHamburger SV, coming on as a substitute forEljero Elia in the 79th minute of a 2–1 home win againstSchalke.
On 7 September 2008, Kačar scored five goals for the Serbian U-21 team in an 8–0 demolition ofHungary in a qualifying match for the2009 UEFA European Under-21 Football Championship.[2] At the main tournament, he also scored once, but it was the only goal that Serbia ever scored.
Kačar's two uncles,Slobodan andTadija, are Olympic medalists in boxing. Slobodan won a gold medal at the1980 Summer Olympics, while Tadija won silver in1976.