He studied at Calle Flygare's drama school in Stockholm and then from 1950 to 1952 at the Royal Dramatic Training Academy; between the two, in the late 1940s, he performed in public parks.

[4] After finishing drama school, Thunberg joined the provincial Östgöta Theatre (Östgötateatern), where he directed in addition to acting, then became a freelance actor.

[5] In the 1950s he became known for his work on the radio programme Mannen i svart ("The Man in Black"), where he presented all kinds of stories of horror and the supernatural in his deep voice, accompanied by a variety of sound effects.

[4] In 1963, he had a supporting role in Ingmar Bergman's drama Winter Light as the church organist Fredrik.

[11] In 1959, Thunberg became engaged to actress Lena Granhagen, whom he had met while directing a play in which she acted in at Intiman.