He was born in Avaldsnes as a son of teacher and farmer Knut Andreas Søndenaa (1871–1948) and Lisa Kristine Thuestad (1880–1958).
As a Mot Dag member, he was a prolific contributor to Arbeidernes leksikon, and Sunnanå followed as the organization became merged into the Labour Party in 1936.
A significant motivation for the project was to please the leftist opposition within the Labour Party in a time of NATO membership and increased spendings on defence.
[2] Satisfying the leftist opposition could not prevent a party split in the long run.
He has been called "the Karl Evang of the fisheries sector", due to their similar career paths: Mot Dag membership, directorate leadership for many years, international organization involvement, involvement in the Kerala project as well as membership of the Labour Party until breaking out in the 1970s.