He began his minor league career in 1960 with the Montgomery Rebels and had stints with the Durham Bulls (1960, 1965), Duluth–Superior Dukes (1961), Knoxville Smokies (1962–63), Syracuse Chiefs (1963–66), Hawaii Islanders (1964), and Toledo Mud Hens (1967–1971).
[4] In October 1968, while playing for the Criollas de Caguas in the Puerto Rican Winter League, Timmermann pitched eight shutouts, including a five-hitter in which he recorded 12 strikeouts among 27 infield outs.
[1] He made his major league debut on June 18, 1969, holding the New York Yankees hitless in 1-1/3 innings as a relief pitcher.
[1] He was credited with converting a bullpen that manager Mayo Smith had called "the worst I've ever seen in baseball" into one that became "almost unbeatable.
[1] On June 15, 1973, the Tigers traded Timmermann to the Cleveland Indians in exchange for Ed Farmer.
[1] He concluded his professional baseball career in 1974 playing for the Toledo Mudhens and Oklahoma City 89ers.