A 2007 junior world champion, Marjanović played abroad in Russia and Lithuania between 2010 and 2012 before making his breakthrough in his home country.

In addition to further MVP awards in Serbia, Marjanović was also voted one of the five best players in the ABA League and the top European club competition EuroLeague in 2015.

He also played for the Detroit Pistons, Los Angeles Clippers, Philadelphia 76ers, Dallas Mavericks and Houston Rockets.

His teammates included Stefan Marković and Milan Mačvan, with whom he had played on the Serbian junior national team.

[14][15] In the first game of the 2014–15 EuroLeague season, Marjanović led his team to a 76–68 victory against Galatasaray, scoring 22 points and pulling down 10 rebounds in 28 minutes on the court.

[17] On November 22, 2014, he recorded 23 points and a career-high 17 rebounds, for a total index rating of 39 in a double overtime 103–110 loss against Galatasaray.

[32] On December 28, in a win over the Minnesota Timberwolves, with Tim Duncan out injured and LaMarcus Aldridge limited to six points, Marjanović scored 17 points on 7-of-7 shooting in 14 minutes to help the Spurs defeat the Timberwolves 101–95 and extend their franchise-record home winning streak to 27 games (dating to 2014–15 season).

[33] Two days later, in a win over the Phoenix Suns, he became the first player in Spurs franchise history to record 12 rebounds in 15 minutes or less.

[38] On April 13, in the team's regular-season finale, Marjanović recorded a career-high 22 points and 12 rebounds in a 96–91 win over the Dallas Mavericks.

He later helped the 76ers get past the Brooklyn Nets in the first round of the 2019 NBA playoffs,[48] but the 76ers were eventually eliminated by the Toronto Raptors in the Conference Semifinals.

[50] On March 11, 2020, Marjanović registered a career-high 31 points, along with 17 rebounds, in a 113–97 win over the Denver Nuggets, in what was the final game before the season was suspended due to the COVID-19 pandemic.

[53] On June 24, 2022, Marjanović was traded alongside Trey Burke, Marquese Chriss, Sterling Brown and the draft rights to Wendell Moore Jr., to the Houston Rockets in exchange for Christian Wood.

[56] On April 14, 2024, in a road game for the Houston Rockets at the Los Angeles Clippers, the Clippers had a partnership with fast food chain Chick-fil-A, where fans in attendance received a free chicken sandwich if any visiting team misses two consecutive free throws in the 4th quarter of a home game.

His senior debut with the Serbian national basketball team at a major tournament came at EuroBasket 2011 in Lithuania, where Serbia finished in eighth place.

At the 2019 FIBA Basketball World Cup, the Serbian national team was considered a favorite to win the trophy,[63] but was eventually upset in the quarter-finals by Argentina.

Marjanović made a cameo appearance as Jānis Krūmiņš in the 2015 Serbian sports drama film We Will Be the World Champions.

In 2021, Marjanović played the grand master in the 1,629th episode of the Serbian television comedy show Državni posao.

Marjanović made an appearance in the Netflix film Hustle, produced by LeBron James and Adam Sandler.