A fiery and outspoken Ontario native, Marchildon went 17-14 for the 1942 Athletics. He became a gunner in the WWII Royal Canadian Air Force, was shot down on his 26th mission in 1944, and lost forty pounds during a nine-month ordeal as a POW in Germany. He returned to baseball in 1945 after recuperating at home. Teammate George Kell said later that “[W]hen he came back he was very quiet and very serious about everything he did. Mr. Mack said he’d really changed. He’d been through a lot. But he said very little about it.”
Marchildon led the last-place 1946 A’s with 13 wins, but tied two teammates with an AL-high 16 losses. In 1947 he helped pull his club out of the cellar with a 19-9 record. He had a perfect game with two outs in the eighth inning in a game against Cleveland when he walked Ken Keltner. Marchildon gave up two singles that tied the score in the ninth, but won his own game with a double in the 12th.