Yarnall was brought into a game against the Brooklyn Robins with the Phillies down 5–2 in the 4th inning. He pitched a scoreless inning, and in the top of the 5th struck out in what would be his only major league plate appearance. During that inning, the Phillies scored four runs to take a 6–5 lead, and Yarnall came out to start the bottom of the 5th. When the first two Robins batters reached base, he was relieved by Jack Knight. Both runners came around to score, and the Phillies lost the game 11–9, with Yarnall taking the loss. He finished the game with one inning pitched, two runs, three hits and one walk allowed, and an earned run average of 18.00.
After his one-game major league career, Yarnall returned to minor league baseball with the Jeannette Jays, and continued to play in the minors on and off until 1933. He also coached baseball, football and basketball while also teaching economics at the Lowell Textile Institute, which later became the Lowell Technological Institute. He became the school's athletic director in 1953, retiring in 1966.
