He chaired his Labour Party chapter from 1945 to 1952, and was first elected mayor of Sandtorg in 1947 and held this office until 1963, when the municipality was incorporated into Harstad.
Bjarne Berg-Sæther chaired several public boards, including Vågsfjord Kraftselskap (1956-1975), the Evenes Airport building committee (1964-1967), Harstad Hospital (1969-1976), the public transportation company Harstad Oppland Rutebil (1964-1986), Fiskernes Bank and later Kreditkassen Bank in Harstad (1975-1986).