Born in Baltimore, Maryland, Thomsen received a Bachelor of Arts degree from Johns Hopkins University in 1919 and a Bachelor of Laws from the University of Maryland School of Law in 1922.

[1] On March 15, 1954, Thomsen was nominated by President Dwight D. Eisenhower to a seat on the United States District Court for the District of Maryland which had been vacated by Judge William Calvin Chesnut.

Thomsen was confirmed by the United States Senate on May 11, 1954, and received his commission on May 12, 1954.

He served as Chief Judge from 1955 to 1970, and as a member of the Judicial Conference of the United States from 1958 to 1964.

In 1968 he presided over the trial of the Catonsville Nine who were charged with burning draft records.