Born in Baltimore in 1907, Thomsen was educated at Baltimore Friends School, graduating in 1926. He then went to Swarthmore College for two years before transferring to St. John's (Md.) College.
While at Friends School, Thomsen played football, basketball and lacrosse, winning varsity letters in all three sports.
At Swarthmore College, he played two years of football and lacrosse, once scoring 14 goals during a lacrosse game versus Lafayette College.
At St. John's College, he also won letters in lacrosse and football and played on the 1929 national championship lacrosse team.
After college, Thomsen played four years with the Mt. Washington Club. Taking up coaching as a career, Thomsen coached from 1930-34 at McDonogh School, then on to Gilman School from 1934-45, where besides coaching, he was also director of athletics for the last six years. In 1945 Thomsen moved to the University of Pennsylvania where he coached lacrosse and football until moving to Princeton University. He coached at Princeton from 1951 to 1970, and it was here that his lacrosse teams won two national championships and 10 Ivy League titles. He won national championships in 1951 and 1953. Thomsen was named Coach-of-the-Year by the USILA in 1967, and he coached the South team in the annual collegiate North-South game in 1950.
In addition to his coaching accolades, Thomsen served an active lacrosse official for 16 years and served several terms as president of the officials association. He held various positions on many committees for the United States Intercollegiate Lacrosse Association and was a former president of the United States Lacrosse Coaches' Association.
Ferris Thomsen passed away in 1994.