He played with the Richmond Colts (1911), Newport News Shipbuilders (1912), Roanoke Tigers (1913), Parksley Spuds (1923–24), and Crisfield Crabbers (1925) over his career in the minors.

[1] In 1909, Ralph Mattis joined the Roxborough semi-professional baseball team, which were members of the Northern Pennsylvania League.

[6] Early in spring training, Mattis suffered an injury which caused him miss playing time.

[7] He was assigned to the Newport News Shipbuilder, where he batted .249 with 107 hits, 18 doubles, four triples, and one home run in 118 games played.

[17] In June, Mattis received a temporary starting job after the Rebels every-day outfielder Davy Jones suffered an injury.

[18] On July 28, Pittsburgh manager Rebel Oakes allowed Mattis to join a semi-professional baseball team in St. Marys, Pennsylvania.

[21] He was competing against Jim Kelly, Mike Menosky, Cy Rheam, Jimmie Savage, and Frank Delahanty for a starting job in the Rebels.

By 1920, Mattis working as a truck driver, and was living in Haverford Township, Delaware County, Pennsylvania, with his wife, Clare E., and their daughter Edna.

[25] During his debut with Parksley on May 27, Mattis forced his team to forfeit in the ninth inning, which The Baltimore Sun called "unsatisfactory".

He finished second amongst league batters in hits, and home runs; third in slugging percentage (.651), and total bases (166); and tied for seventh in doubles.

Before the start of the 1924 season, Mattis resigned as the manager for the Parksley Spuds, stating that he wanted to devote all of his time to playing.

[27] On June 24, in a game against the Cambridge Canners, Mattis hit a home run during the bottom of the 12th inning to give the Spuds the 2–to–1 victory.

Mattis finished the season third in the league in home runs, and total bases (194); and fifth in hits, and slugging percentage (.554).

In 1942, Mattis was living in Radnor Township, Delaware County, Pennsylvania, and working for the Philadelphia Electric Company.