From 1962 he was an actor in Sweden's prominent Royal Dramatic Theatre, where he came to perform a number of much celebrated parts: his eccentric Hitler in Schweik in the Second World War by Bertolt Brecht (1963), Estragon in the legendary 1966 Dramaten-staging of Samuel Beckett's Waiting for Godot, Thersites in Shakespeare's Troilus and Cressida 1967, Orgon in Molière's Tartuffe 1971, Hjalmar Ekdahl in Ingmar Bergman's 1972 production of Ibsen's The Wild Duck, Nero in Jean Racine's Britannicus (1974), a spot-on portrayal of August Strindberg in play Tribadernas natt (The Night of the Tribades) by Per Olov Enquist, the title role in Richard III by Shakespeare (1980) and the extremely creepy – and slightly perverted – boss Sven in VD ("CEO") by Stig Larsson in 1985, among others.

Adding the fact that Järegård also had a beautiful and expressive singing voice (he performed in a number of stage musicals during his career) gave him an incredible range and versatility as an actor.

Particularly popular are his audio book (originally radio) recordings of Roald Dahl's Charlie and the Chocolate Factory, Robert Louis Stevenson's Dr Jekyll and Mr Hyde, and the narration of Sergei Prokofiev's Peter and the Wolf.

He participated in about 20 movies and 40 TV productions: aside from the Riget-series; some of his greatest roles are in the Skånska mord-series, in the 1975 Hasse & Tage comedy Släpp fångarne loss, det är vår!

Von Trier claims that he eventually 'trained' Järegård by rewarding him with cigars for good behaviour but also called the actor "dear Ernst-Hugo" and said he misses him terribly.