Mattis was invited to spring training with the Washington Senators in 1912.[6] Early in spring training, Mattis suffered an injury which caused him miss playing time.[6] The manager of the Georgetown University baseball team allowed Mattis to practice with his team while he was recovering from his injury.[6] Before the start of the regular season, Mattis was sent to the Class-C Virginia League by the Senators.[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. At the end of the season, The Sheboygan Press called Mattis "the best outfielder in the Virginia League".[8] In 1913, Mattis joined the Class-C Roanoke Tigers of the Virginia League. During mid-season, he was described as one of the best outfielders in the Virginia League by The Washington Post.[9] He batted .300 with 157 hits, 21 doubles, eight triples, and three home runs in 140 games played. Defensively, he played all of his games in the outfield, committing nine errors in 332 total chances. Mattis led the league in hits that season.[10]
On February 25, 1914, Mattis signed with the major league Pittsburgh Rebels of the newly formed Federal League.[11][12] He attended spring training with the Rebels that season in Lynchburg, Virginia.[13]The Pittsburgh Press described Mattis as having "shoulders like an ox" and that it is likely he would be a star player for Pittsburgh.[14] The Sporting Life predicted that he would develop into one of the best outfielder is the Federal League.[15] He made his MLB debut on April 22, against the Brooklyn Tip-Tops.[16] In his first career MLB plate appearance during that game, he drew a walk.[16] His first hit came that day, as he got a single, double, and triple in three at-bats.[16] After two months, Mattis was second in the Federal League in batting average with a .406 clip.[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 managerRebel Oakes allowed Mattis to join a semi-professional baseball team in St. Marys, Pennsylvania.[19] He returned to the Rebels that season on September 11.[20] On the season with Pittsburgh, he batted .247 with 14 runs scored, 21 hits, four doubles, one triple, eight runs batted in (RBIs), and two stolen bases in 36 games played. Defensively, Mattis made three errors in 43 total chances in the outfield.
In 1923, after a nine-year absence, Mattis made his return to professional baseball. He signed with the Class-D Parksley Spuds of the Eastern Shore League.[25] He served as the Spuds' player-manager.[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".[25] He finished the season with a .369 batting average, which was third in the league.[26] Mattis also compiled 94 hits, 13 doubles, one triples, and 19 home runs in 65 games played. 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.[26] Defensively, Mattis played 64 games in the outfield, committing three errors in 133 total chances.
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.[27] On the season, Mattis batted .322 with 91 hits, eight doubles, two triples, and 14 home runs in 73 games played. He finished fifth in the league in batting average, and home runs; and sixth in hits.[28] During all of his games, he appeared as an outfielder. He primarily played center field that season.[29]
Mattis signed with the Class-D Crisfield Crabbers of the Eastern Shore League in 1925. In 90 games played, Mattis batted .311 with 109 hits, 14 doubles, one triple, and 23 home runs. Defensively, he played all of his 90 games in the outfield. He committed four errors in 162 total chances. Mattis finished the season third in the league in home runs, and total bases (194); and fifth in hits, and slugging percentage (.554).[30] That season would later prove to be his final in professional baseball.
