She is a two-time world champion and double Olympic medallist who ranks as the joint third- highest female jumper of all time with her personal best of 2.08 m (6 ft 9+3⁄4 in).

The daughter of Croatian decathlon record holder Joško Vlašić, she was a talented junior athlete and attended her first Olympic Games in 2000 Sydney at the age of sixteen.

From a young age, she was involved in sports: her mother Venera was a seasoned amateur in basketball and cross-country skiing while her father, Joško Vlašić, was an international athlete who broke the Croatian record in the decathlon.

[7] Vlašić had an early start in international competition: she competed at the inaugural World Youth Championships, finishing eighth,[8] and represented her country for the first time at the 2000 Sydney Olympics.

[10] She rounded off the year by winning her first senior gold medal at an international tournament, taking first place at the 2001 Mediterranean Games.

[7][14] June and July yielded further progress, jumping 1.98 m again and improving to 1.99 m to win her first IAAF Golden League event at the Gaz de France.

[6] Although her poor health had spoiled her medals chances at the two major championships of 2004 and 2005, Vlašić came back fully recovered and stronger in the 2006 season.

"[21] The 2006 European Athletics Championships in Gothenburg proved to be a bittersweet experience: she cleared 2.01 m to finish in fourth place, behind Tia Hellebaut, Venelina Veneva and Kajsa Bergqvist.

[6][16] Vlašić capped the season off with an appearance at the 2006 World Athletics Final but withdrew from the competition after her third jump, finishing sixth.

Having won the previous five Golden League events, she finished the Memorial Van Damme meeting in second place behind Ariane Friedrich.

During the final, the duo was joined by reigning world silver and Olympic bronze medalist Anna Chicherova, who took the lead with a 2.02 m first-time clearance.

The victory at the Hochsprung mit Musik added one centimetre to her own personal best and Croatian record and brought her to third on the all-time indoor lists[36] behind Sweden's Kajsa Bergqvist (2.08 m in 2006) and Germany's Heike Henkel (2,07 m in 1992).

On 1 August 2010 Vlašić won her first European title with 2.03 m, equalling the championship record set by Belgian's Tia Hellebaut and Bulgaria's Venelina Veneva-Mateeva four years earlier at the 2006 edition in Gothenburg.

With 2.00 m, she wasn't leading the world rankings and Russian long time rival Anna Chicherova had all the expectations for the win, thanks to a Russian record of 2.07 m. Italy's Antonietta Di Martino is also in a good shape coming to the Worlds, having (as Vlašić) jumped 2.00 m outdoors but did better indoors with an Italian record of 2.04 m. Moreover, Vlašić was not guaranteed to show up at the championships due to a left leg injury.

But Anna Chicherova beat the Croatian on countbacks, while Antonietta Di Martino has to settle for the bronze with 2.00 m.[40] Vlašić decided to have an operation on her left Achilles tendon in January 2012.

She placed 6th with 1.94 m.[44] Due to a jumper's knee injury in her left knee,[45] she had to withdraw from the European championships in Zürich[46] where she was one of the favorites alongside Russia's Mariya Kuchina, due to her two recent wins in Paris and London with 2.00 m.[47][48] However, she came back on the scene a week after the Europeans at the Weltklasse Zürich and placed fourth with 1.93 m.[49] She ended her season on a good note, jumping that height again in Zagreb.

In 2015, Vlašić opened her outdoor season at the Golden Gala in Rome and placed 2nd to Spain's Ruth Beitia (2.00 m, world lead) with 1.97 m. Then she equaled that height in New York City, again beaten by the Spaniard on countbacks.

However, due to her chronic foot pain, she cancelled her appearances in both Lausanne[50] and Monaco in order to get ready for the Beijing World Championships.

On 27 August, she made it to the finals and there, two days later, earned the silver medal behind Russia's Mariya Lasitskene (former Kuchina) who beat her on countbacks (Vlašić had one miss at 1.92 m).

[51] To her, the silver medal felt like gold after all she had to go through in the previous years, and more specifically recalling she could not walk for days in early July.

[52] Still due to her Achilles problem, Vlašić only made one appearance in the 2016 season in Split on January 29 where she took the win with 1.95 m, jumping over the qualifying standard (1.93 m) for the 2016 Summer Olympics.

[54][55] Despite having not competed during the outdoor season (she cancelled her participation at the European Championships and in the London Diamond League), she was selected by the Croatian Federation alongside Ana Šimić to represent Croatia in the high jump at the Olympics.

[65] Vlašić was named after Casablanca, a city where her father competed and won a gold medal at the 1983 Mediterranean Games around the time of her birth.

[71][72][73][74][75] On 26 May 2022 Vlašić announced on instagram that she had married Belgian sports journalist Ruben Van Gucht and that she was pregnant with the couple's child.