After attending the University of Santa Clara, he became a professional baseball player during World War I and was acquired by the Chicago White Sox at age 19 during the 1918 season.

Although Shellenback posted a 7–3 record there, because of his minor league status, he was not included on a list of "grandfathered" spitball hurlers who would be allowed to continue to use the pitch at the major-league level.

[2] Shellenback's long tenure with Los Angeles-based teams even led to a brief movie career; he had roles in the comedies Fireman, Save My Child (1932) and Alibi Ike (1935).

There, in 1936, he helped discover and groom one of the greatest hitters of all time, Ted Williams, who signed with the Padres as a 17-year-old pitcher-outfielder out of San Diego's Hoover High School.

He became a Giants scout and roving minor league pitching instructor in 1956 and continued in that role until his death at age 70 in Newton, Massachusetts.