Stark turned professional a year later when he signed with the Scottish-Americans of the National Association Football League for the 1912–1913 season.
In 1915, the Scottish-Americans won the 1915 American Cup, defeating Brooklyn Celtic 1–0 on a Stark goal.
In 1917, the U.S. entered World War I, and Stark joined the U.S. Army, which briefly interrupting his career.
[2] When he returned to the U.S. in 1919, he joined Paterson F.C., which went to the 1919 National Challenge Cup final, losing to Bethlehem Steel 2–0.
Following the cup, Stark joined Bethlehem for the team's August 10, 1919, to September 24, 1919, tour of Sweden and Denmark.
In a controversial decision, league officials awarded Erie A.A. the title even though Bethlehem Steel had one more point.
Bethlehem had some of the deepest pockets in the ASL and used that money to sign several top Scottish internationals.
Surrounded by talent, Stark responded by scoring 67 goals in 44 games in his first season with the team, including eight hat-tricks,[5] which is the correct World Record for an individual.
[6] Reasons for this centered on a dispute between the ASL and the U.S. Football Association (USFA), which was backed by FIFA, about control of the sport in the United States.
However, the team faced financial difficulties and left the players stranded in Budapest, after which they returned to the U.S. as third class passengers.
When Bethlehem Steel folded in 1930, Stark moved to the Newark Americans for the 1930–1931 ASL season.
In November, Canada came to the U.S. for a game; Stark scored five goals as the U.S. easily dispatched its northern neighbors 6–1.