A keen amateur ice hockey player in his youth, he also joined theatre teacher Ingemar Lind's Institute for the Performing Arts in the village of Storhögen outside Östersund.

There Lassgård met the director Peter Oskarson and joined his Skånska Teatern theatre company at Landskrona, where he remained for four years, making his first television appearance as "Puck" in its production of A Midsummer Night's Dream in 1980.

He has gone on to play in a wide range of roles in films, notably as policeman Gunvald Larsson in a series of films made in 1993–94, based on the Martin Beck novels, and starring Gösta Ekman, and as Kurt Wallander in the SVT TV-movie adaptations of the Henning Mankell novels from 1994 to 2007.

[4] From 2011 to 2015, Lassgård has appeared in Seasons 2, 3, and 4 of the TV Norwegian comedy series Dag, playing a free-thinking, free-wheeling therapist.

In 2013 he starred in the crime series The Death of a Pilgrim, a dramatic retelling of the assassination of Olof Palme and fictionalised account of the discovery of his killer in the 2010s.