Thomasson came up for a cup-of-coffee in 1972 for his first taste of the Big Leagues, appearing in 10 games for the San Francisco Giants and going 9-for27 for a .333 average.
In 1973 he would play in 112 games, putting in a nice rookie year, batting .285 with 67 hits over 235 official at-bats, scoring 35 runs while driving in 30.
Though he wouldn’t hit that high again during his career, he would pretty much put up the same numbers every year, becoming a solid platoon player, only having a truly full season of action once over the length of his career, in 1977 when he appeared in 145 games and had 534 plate appearances, hitting a career high 17 homers while driving in 71 runs.
After six seasons in San Francisco, he was part of the trade that brought the Giants pitching ace Vida Blue, with Thomasson and six others going to Oakland, where he would play through June before getting traded yet again, this time East to the eventual World Champion New York Yankees for Dell Alston and Mickey Klutts.
The trade allowed him to see the only Postseason action he’d get in his career, going 1-for-4 against the Dodgers, ironically the team he’d find himself playing for the very next season, where he would play out his active career in 1980, finishing up with a .249 batting average with 591 hits over 2373 at-bats and exactly 900 games.