Signed by the Giants as a bonus baby, McCormick went directly to the major leagues and made his debut on September 3, 1956. After spending seven seasons with the organization, he was traded to the Baltimore Orioles and played two years there before being dealt again, this time to the Washington Senators. He returned to the Giants in 1967 and in his first season back with the team, he became the first pitcher in Giants franchise history to win the Cy Young Award. In the middle of the 1970 season, he was traded to the New York Yankees, who released him before the start of the next season. He then signed with the Kansas City Royals, with whom he played his last game on May 22, 1971.