In 2006, he landed his first role in a movie, when he joined the cast of the Norwegian film Reprise, where he played Morten.

In 2012, Williamson portrayed the Norwegian painter and sailor Erik Hesselberg, one of the members of Thor Heyerdahl's Kon-Tiki expedition, in a film of the same name.

The same year, he began appearing in the miniseries Nobel, where he plays the soldier Hans Ivar Johansen.

As of 2019, Williamson appears on the HBO show Beforeigners, where he plays the corrupt police officer Jeppe.

[5] Williamson married Tinashe Bakas Roll in 2015, and they have one child together, a girl named Ziggy.