The Dayton Triangles were an original franchise of the American Professional Football Association (now the National Football League (NFL)) in 1920. The Triangles were based in Dayton, Ohio, and took their nickname from their home field, Triangle Park, which was located at the confluence of the Great Miami and Stillwater Rivers in north Dayton.
The original Dayton Triangles members first began playing together as basketball players at St. Mary's College, now the University of Dayton, from 1908 until 1912. After graduation, the players organized a basketball team of alumni, students, and other local athletes. They went by the name of the St. Mary's Cadets. The Cadets claimed the title of "World Basketball Champions" by defeating the Buffalo German Ramblers.
In the fall of 1913, the St. Marys Cadets organized a football team. The team was coached by Louis Clark, who coached the St. Mary's college football team as well. Al Mahrt was elected team captain. The team finished its first season with a 7–0 record and won the Dayton City Championship. It also won the Southern Ohio Championship by defeating the Cincinnati Celts 27–0 at Redland Park. The team won a second city championship in 1914, despite injuries to Al Mahrt and Babe Zimmerman. In 1915 the team changed its name to the Dayton Gym-Cadets after their presumed sponsors, the Dayton Gymnastic Club. That season saw Al Marhrt take over as the team's coach. The team only lost one game that season, to the Columbus Panhandles. It also won its third city championship.