Merritt originally signed with the Los Angeles Dodgers out of Edgewood High School in West Covina, California, but was plucked from their organization by the Minnesota Twins in the 1961 first-year draft before pitching a game for his home team.

He went 54–45 with a 3.39 earned run average and 719 strikeouts over four seasons in their farm system when he was called up to the majors in 1965 by the first place Twins just before World Series rosters were set.

He turned his season around on July 21 with a three hit gem against the Washington Senators in which he struck out twelve and matched an American League record with seven strikeouts in a row.

[2] The Reds battled the Dodgers, Atlanta Braves and San Francisco Giants for first place in the National League West all through the 1969 season.

Merritt's sixteenth victory on August 31[3] moved his team into a second place tie with the Dodgers, a half game back of the Giants.

[5] His record stood at 14–7 with a 3.46 ERA at the 1970 All-Star break to earn selection to Gil Hodges' National League squad.

[6] He finished the season at 20–12, and edging out Gary Nolan for the Johnny Vander Meer Award given each year to the Reds' top pitcher.

Perhaps the most notable moment of his Texas Rangers career occurred on August 23, 1973, when Merritt credited the three hit shutout he pitched against the Cleveland Indians to the 25 "Gaylord fastballs" he threw.