Born in Mostar, Mladenović passed the youth school of Sloga Kraljevo. He made his debut for the first team of same club during the 2008–09 season, making 2 Serbian League West appearances. As he did not play in the Serbian First League next season, six months before the end of youth career, he was loaned to neighbourhood, and spent spring half of the 2009–10 season with Metalac Trgovački, playing in the Serbian League West.
Siniša spent first match of the season on the bench, but after Jovica Vasilić extended loan at Sloboda Užice, Mladenović got the chance in first squad from the 2 fixture match against Teleoptik, played on the Kraljevo City Stadium. In next fixture, against Mladost Lučani he got red card after 20 minutes of the game and suspension for next round. Later, during the season, Mladenović was mostly standard on the right-back position. He missed several matches between 23 and 25 fixture. In 26 fixture away match against Napredak Kruševac, Mladenović was substituted in for Rašo Babić in the half-time.
Mladenović started 2012–13 season in the first squad. After first match against Kolubara, played away, he missed home match against Borac Čačak in the 2 fixture, when Veljko Antonijević played on that position. Later, he played all matches until the end of first half-season, including cup match against OFK Beograd, when he replaced Dušan Mladenović in second half of that match. He started 2 matches from the bench, against Jedinstvo Putevi, and Radnički Nova Pazova. In the first spring fixture, against Kolubara, Mladenović got the red card after one duel with Saša Jovanović. Previously, he moved on left-back position, because of Miloš Milovanović's injury. Later, coach Neško Milovanović converted Marko Gobeljić to right-back, so Siniša moved on the bench. For the rest of season, he made 8 appearances, just two times the whole 90 minutes.
Coming Veljko Dovedan for the coach of Sloga Kraljevo, Mladenović was often changing position on the field for the 2013–14 season. Gobeljić continued playing as a right-back, so Mladenović started season on the bench as a reserve player, usually replacing some of players during the game. Whereas Sloga started season with a pair of standard stoppers, Miletić & Drašković and a few of inexperienced players, Mladenović also played as a centre-back in some occasions. He made his first season start in away match against Radnik Surdulica. Later, he was mostly used as a left-wing and playing on that position he scored his only goal for the team in last fixture of the first half-season, against Proleter Novi Sad. In away match against Metalac Gornji Milanovac, Mladenović was paired with Danijel Gašić on stopper places, because Nemanja Miletić's yellow cards accumulation. He also played appeared as a left-back in a match against Inđija, replacing suspended Miloš Milovanović. After that match Dovedan left the club, and Igor Tufegdžić returned Mladenović on his original right-back position for the rest of season. After he made a mistake in a match against Borac Čačak, Milan Jevtović scored a goal for an away team, and Mladenović was replaced in the second half by Marko Gobeljić. Next the end of a season, Mladenović left the club.
