Johnson was signed by the New York Mets as an amateur free agentthird baseman in. He batted.248 for their Western Carolina League affiliate, the Salisbury Braves in 1962, and committed seven errors in only 29 games for a.868 fielding percentage. He batted.238 with an.872 fielding percentage in, and the Mets began experimenting with him on the mound. He made eleven pitching appearances for the California League's Salinas Mets that season, and had a 6.75 earned run average without a decision. With an.870 fielding percentage in, the notion of Johnson as a third baseman was abandoned, and he was converted to a pitcher for good by the New York–Penn League's Auburn Mets. He pitched in the Mets' minor league system through. Following the season, he was obtained by the Philadelphia Phillies from the Mets in the 1967 minor league draft.
Johnson spent two years with Philadelphia, before moving to the St. Louis Cardinals in a controversial transaction. On October 7,, he was sent by Philadelphia along with Dick Allen and Cookie Rojas to the St. Louis Cardinals in the same trade that brought Tim McCarver, Byron Browne, Joe Horner and Curt Flood to the Phillies. After Flood refused to report to his new team, St. Louis sent Willie Montañez and a minor leaguer to Philadelphia to complete the trade. Flood believed that Major League Baseball's reserve clause was unfair and appealed his case to the U.S. Supreme Court. Although his legal challenge was unsuccessful, it brought solidarity among ballplayers as they fought against reserve clause and sought free agency.
He was 2-0 with a 3.18 ERA for the Cards when they dealt him mid-season to the San Francisco Giants for Frank Linzy. With the Giants, Johnson emerged as one of the top relievers in the National League, finishing sixth in Cy Young Award balloting during an era when such an honor was rare for relief pitchers. In, Johnson went 12-9 with a 2.97 ERA and eighteen saves for the division winning Giants. In his only post-season, he pitched 1.1 innings and gave up two runs to the Pittsburgh Pirates in game four of the 1971 National League Championship Series.
Later career
During Spring training, Johnson was selected off waivers by the Cleveland Indians. After one season in the American League, he was traded to the Houston Astros for Cecil Upshaw. He was released by the Astros at the end of the season, and signed with the San Diego Padres in. After two seasons in San Diego, he was traded to the newly created Toronto Blue Jays for Dave Roberts, who had just been purchased by the Jays from the Padres. Johnson was the winning pitcher in the Blue Jays' first-ever game on April 7, 1977. Johnson went 2-4 with a 4.60 ERA in the Blue Jays' inaugural season, at the end of which he retired. Following his majors career, Johnson played for the St. Lucie Legends of the Senior Professional Baseball Association.
