Kostner has won 11 medals at the European championships, most recently in 2018, and is the most decorated singles skater in the history of that competition.

Making her senior international debut, Kostner won gold at two September events, the 2002 Nebelhorn Trophy and 2002 Ondrej Nepela Memorial.

Ranked first in her qualifying group, first in the short program, and fifth in the free skate, she took bronze at the 2003 World Junior Championships in Ostrava, Czech Republic.

[19] Kostner won her first European medal in 2006, and was chosen to be flag bearer for the host Italian team during the opening ceremony of the 2006 Winter Olympics.

[23] At the 2008 Worlds, Kostner won the short program and placed third in the free skate, winning the silver medal overall.

[28] Kostner placed 6th at both of her Grand Prix events, the 2009 Trophée Eric Bompard and the 2009 Cup of China.

In the middle of the season, she left Carroll but continued training with Christa Fassi,[29] and in Pinerolo, Italy with Edoardo De Bernardis.

In December 2009, Kostner lost her national title to Valentina Marchei, which threatened her spot on the Italian team for the Vancouver Games, but the following month she rebounded to win gold at the 2010 European Championships, held in Tallinn, Estonia.

At the 2010–11 Grand Prix Final, Kostner placed second in the short program and fourth in the long, winning the silver medal.

At the 2011 Worlds in Moscow, Kostner was sixth in the short program but won the bronze medal after a personal-best free skate.

She was the silver medalist at Skate America and won the Cup of China, thus becoming the first skater to qualify for the Grand Prix Final.

In an interview after the event, she stated that her knee was fully recovered and her goal was to include more difficult jumps in the 2012 ISU championships.

[36] She is the first Italian single skater to become a Grand Prix Final champion and the second overall after Barbara Fusar-Poli / Maurizio Margaglio, who won the ice dancing title in 2000.

[38] At the 2012 World Championships in Nice, France, Kostner finished third in the short program[39] and first in the free skate with a new personal best score to take the gold.

[46] On 2 August 2012 her name was removed from the entry lists of both of her 2012–2013 Grand Prix events[47] due to insufficient time to reach competitive fitness.

At the 2014 European Championships in Budapest, Hungary, Kostner won the bronze medal, her 9th podium in a row in the continental competition.

"[51] One month later, at the 2014 World Championships in Saitama, Japan, Kostner placed second in the short program with a score of 77.24, her personal best.

In January 2015, Kostner received a 16-month suspension from competition for lying about the location of her ex-boyfriend Alex Schwazer, reportedly to help him conceal his use of an illegal performance-enhancing hypobaric chamber machine and to help him avoid a drug test.

[53][54] On 5 October 2015, the Court of Arbitration for Sport (CAS) announced Kostner had agreed to a demand by Italian doping officials to increase the suspension to 21 months but to backdate the start of the ban to 1 April 2014.

In an interview, she announced her intention to compete at the World Championships, held in Milan, and stated that she had not yet decided when she would retire from competitive skating.

[58] Kostner skated a new personal best score of 80.27 in the short program at the 2018 World Championships, taking first place.

[61] At the end of December, she broke the silence to announce that she was still struggling with the after effects of her hip injury and wouldn't go to the Europeans, but aimed at appearing at the Worlds in March.

[64] Skaters she has worked alongside include Nicole Schott,[65][66] Lee Hae-in,[67] Yuma Kagiyama,[68][69] and Rebecca Ghilardi/Filippo Ambrosini.

Kostner's current and former official suppliers[72] include Maybelline, L'Oréal Professionnel, Fratelli Rossetti, Damiani, Swarovski, Iceberg, and T-SHIRT T-SHOPS.