He was born in Butte, Montana on December 17, 1916, to Anna (née Vennes) and Albert Olsen, both Norwegian immigrants.

[5] In 1960, when Congressman Lee Metcalf opted to run for the Senate rather than seek re-election, Olsen ran to succeed him in the 1st congressional district.

When Olsen ran for a sixth term in 1970, he was narrowly defeated for re-election by Richard G. Shoup, the Mayor of Missoula.

In 1974, he ran for Congress one final time, but lost in the Democratic primary to Max Baucus, who ended up defeating Shoup in the general election.

Olsen was appointed by Governor Thomas Lee Judge to the Second Judicial District of Montana, in 1977, and elected to that post in 1979 and two additional times, serving until his death in 1990.