[2] He made his Major League debut on September 5, 1972, at the age of 21, pinch-hitting for pitcher Frank Reberger in a 4–3 Giants' win over the San Diego Padres.

Thomasson was traded along with Gary Alexander, Dave Heaverlo, John Henry Johnson, Phil Huffman, Alan Wirth and $300,000 from the Giants to the Athletics for Vida Blue on March 15, 1978.

Thomasson spent only a few months and 47 games with Oakland before being traded to the New York Yankees for Dell Alston, Mickey Klutts, and $50,000, on June 15, 1978.

Purchased from the Dodgers by the Yomiuri Giants of Japanese Nippon Pro Baseball on December 22, 1980, Thomasson spent his final two professional seasons (1981–82) in Japan.

[citation needed] Tokyo writer and conceptual artist Genpei Akasegawa published a book containing photographs of found objects which he termed "Hyperart Thomasson".