He was born in Porsgrunn as a son of ship captain Aksel Zachariassen (1853–1927) from Luleå and Karen Nilsen (1860–1933) from Ulefoss. He began his journalistic career in Bratsberg-Demokraten. In 1920 he was hired in the Det socialdemokratiske Pressekontor (where he met his wife), he went on to Arbeiderbladet in 1923, Halden Arbeiderblad in 1928, Arbeidermagasinet in 1931 and Kongsvinger Arbeiderblad in 1932. In the latter newspaper he was the editor-in-chief. He was also involved in revolutionary politics, and headed the Young Communist League of Norway from 1921 to 1923, while the organization was still affiliated with the Norwegian Labour Party. Also, he was subject to deportation from the United Kingdom during a visit there in 1919, where he among others tried to rally support for the Red Guards of Finland, and met with Sylvia Pankhurst. Zachariassen participated in the Left Communist Youth League's military strike action of 1924. He was convicted for assisting in this crime and sentenced to 90 days of prison.
