Messersmith is most famous for his role in the historic 1975 Seitz decision which led to the downfall of Major League Baseball's reserve clause and ushered in the current era of free agency.

"It was less of an economic issue at the time than a fight for the right to have control over your own destiny", Messersmith told The Sporting News, looking back on his decision a decade later.

Among other things, then-Braves owner Ted Turner suggested the nickname "Channel" for Messersmith and jersey number 17, in order to promote the television station that aired Braves games.

Messersmith struggled trying to live up to his new contract which was sold to the New York Yankees for $100,000 at the Winter Meetings on December 8, 1977,[1] having gone 16–15 in two seasons with the Braves, the second marred by injuries.

The Yankees released him after an injury plagued 1978 season and he signed with the Dodgers, who gave him the very thing their first refusal drove him toward testing and defeating the old reserve system: a no-trade clause.

Messersmith's earned run average of 2.861 is the sixth lowest among starting pitchers whose careers began after the advent of the live-ball era in 1920, behind only Clayton Kershaw (2.44), Hoyt Wilhelm (2.52), Whitey Ford (2.75), Sandy Koufax (2.76), and Jim Palmer (2.856).