He negotiated Norway's trading deal with European Economic Community in 1972 as minister of commerce in which he served in the governments of both Trygve Bratteli and Odvar Nordli.

During the German occupation of Norway, Evensen volunteered in the Norwegian resistance movement, helping, among other things, to create false identity papers.

After World War II, he was appointed attorney in fact and prosecutor a number of treasons trials the Norwegian government brought against collaborators during the post-war legal purge.

Former prime ministers Odvar Nordli and Kaare Willoch praised Evensen's work on securing Norway's rights to offshore resources, which, in turn, spawned the country's oil industry.

He was both respected and controversial and angered fellow Labour Party officials when he agreed to shared management of fishing resources in the Barents Sea with the Soviet Union.

He remained an international expert on offshore rights and contributed to the creation of economic zones extending 200 nautical miles (370 km) out to sea.