Šimić joined Dinamo Zagreb in 1987 at the age of twelve and rose through the ranks of their youth academy before being promoted to the senior squad for the 1992–93 season.

Other notable Dinamo players and Šimić's teammates from that era were Silvio Marić, Dražen Ladić, Igor Cvitanović, Mark Viduka and Robert Prosinečki, as well as Krunoslav Jurčić and Goran Jurić with whom he formed a solid defensive partnership.

In 1996, impressed by his string of good performances, Blažević called him up and, after playing 16 games for Croatia's youth teams, Šimić earned his first full international cap on 13 March 1996 in a friendly against South Korea, coming on as a substitute for Igor Štimac.

The club failed to win any silverware in this period, and their highest finish was fourth place in the 1999–2000 season, even though this was the time when some of football's biggest stars played there with Šimić, such as Roberto Baggio, Ronaldo, Ivan Zamorano, Diego Simeone and Christian Vieri.

Šimić went on to make 12 appearances for AC Milan in their run to the UEFA Champions League title in 2003, though he was omitted from both semi-final legs against Inter and the final match against Serie A rivals Juventus at Old Trafford.

In May 2006, he signed a contract extension until 2009 with AC Milan, just weeks after openly criticising coach Carlo Ancelotti over lack of playing time.

In his only UEFA Champions League appearance that season, against Celtic, he was member of the starting eleven but was forced to leave the pitch due to an injury after spending less than 30 minutes in the game.

[8] He made his Ligue 1 debut on 23 August 2008 in Monaco's 1–1 draw at home to Caen, playing the entire match and assisting Frédéric Nimani for the opening goal.

He lost his place as a regular in the team after that, making no appearances since Monaco's final league match of the 2008–09 season, a goalless draw at Paris Saint-Germain.

[13] In his announcement, Šimić said that he enrolled at the coaching academy and confirmed that his future plans involved staying in football, possibly as a manager.

He made his senior international debut on 13 March 1996 in a friendly match against the Korea Republic, coming on as a substitute for Igor Štimac in the final 20 minutes.

However, he missed the first two games of the tournament, unable to dislodge Vedran Ćorluka, Robert Kovač or Josip Šimunić from their berths in the first team.

Šimić was subsequently also called up for Croatia's opening two qualifiers for the 2010 FIFA World Cup in early September 2008, at home to Kazakhstan and England, being an unused substitute in the former and completely left out of the latter.