Sim Mathiesen began an illustrious athletic career at Fresno High School, participating in football, basketball, and baseball from 1926 through 1928. Mathiesen continued his athletic career at Fresno State from 1928 through 1931. He became one of the finest punters on the West Coast at the college or university level. Mathiesen averaged forty-five yards per kick for an unbeaten 1930 Bulldog team coached by Stan Borleske. He was an All-Far Western Conference halfback in 1929 and 1930. Mathiesen was a three-year varsity basketball letterman and team captain in his final two years. Showing his versatility, Mathiesen played on the Fresno State golf team for three years and, after his college days, won the Fresno City Golf Championship in 1943. In 1934, he became the first football coach at Sierra High School near Auberry and continued there through 1935. From 1936 to 1943, he served as Supervisor of Physical Education for Fresno County Schools. Mathiesen's son, Mike, was an outstanding baseball player at Fresno State and also professionally. His wife, Mickey, was one of the top women amateur golfers for many years.