As an 18-year-old goaltender, Fasth played with the Brooklyn Tigers (a youth affiliate connected to Luleå HF[2]) in the Swedish Division 2, the fourth-tier league.

He was set to be the team's backup goaltender, but injury problems for Christopher Heino-Lindberg forced AIK to start Viktor Fasth.

He significantly contributed to AIK's success that season, reaching the semifinals in the playoffs, where they were eliminated by Färjestad BK in four games.

On 21 May 2012, Fasth signed a 1-year, one-way $1 million USD deal with the Anaheim Ducks of the National Hockey League (NHL).

[5][6] On 20 September 2012, due to the ongoing 2012–13 NHL lockout, Fasth signed to play with his former team Tingsryds AIF of the HockeyAllsvenskan (Swe-2).

Fasth played in 12 games for the team, registering an impressive 94.17% save percentage and a 1.68 goals against average (GAA).

[7] In his NHL debut on 26 January 2013, Fasth led the Ducks to a 3–2 shootout victory over the Nashville Predators.

On 4 March 2014, Fasth was traded by the Ducks to the Edmonton Oilers in exchange for a fifth and a third round pick in the 2014 and 2015 NHL Entry Draft, respectively.

[9] On 9 July 2015, Fasth agreed to terms on an initial one-year contract with CSKA Moscow of the Kontinental Hockey League (KHL).

As a free agent following the 2016–17 season, Fasth opted to return to his homeland and agreed to a second stint with Växjö Lakers, who were promoted to the SHL in his absence from leaving the club in 2010.

[13] In the World Championship tournament he helped Sweden reach the final and led the leading goaltenders league until the gold medal game against Finland.