He played in Major League Baseball for the New York Yankees, Oakland Athletics, Milwaukee Brewers, and Los Angeles Dodgers from 1961 through 1977.

[3] Downing signed with the New York Yankees as an amateur free agent in 1961, and was promoted to the major league roster by July of that season.

In 1963, his first full major league season, Downing had a 13–5 win–loss record with a 2.56 earned run average (ERA) for a Yankee team that went 104–57, but were swept by the Los Angeles Dodgers in the 1963 World Series.

He was traded to the Oakland Athletics prior to the 1970 along with catcher Frank Fernández for Danny Cater and Ossie Chavarria.

[7] In his first season in the National League (NL), Downing won twenty games, and pitched a league-leading five shutouts.

[8] On April 8, 1974, Downing allowed a home run to Hank Aaron that was the 715th of his career, breaking the all-time record set by Babe Ruth.