He was a member of the communist movement Mot Dag before the German occupation of Norway 1940–1945, during which he had to flee the country.

In Stockholm he edited the underground newspaper Håndslag, together with Eyvind Johnson and Willy Brandt.

Funded by the Norwegian legation in Stockholm, it was issued once every second week, and was printed in 15–20,000 copies towards the end of the war.

During this time he also wrote several non-fiction books, mainly political literature: Frihet og demokrati (Freedom and Democracy, 1947), Øst og Vest (East and West, 1948), Sosialismen under debatt (Socialism Under Debate, 1950) and Sovjetmysteriet (The Soviet Mystery, 1957).

[2] Elster was awarded the Riverton Prize for the crime novel Thomas Pihls annen lov (1982).