As a player Merrill was listed as tall and ; he batted left-handed and threw right-handed. He began his baseball career as a
catcher for the
Maine Black Bears, where he also played
football and earned a degree in
physical education. He was selected by the
Philadelphia Phillies in the
free-agent draft. He spent that season in the class-A
New York–Penn League with the
Batavia Clippers, and the following year with single-A teams in
Bakersfield and
Eugene. In and , Merrill was at the double-A level with the
Reading Phillies, and returned to Eugene in and while that team was at the triple-A level. His playing career ended after the 1971 season due to a leg injury.
As a manager Merrill's managerial career started in with the
West Haven Yankees of the
Eastern League. He led the team to the best overall record in the league that season, and to a first-place finish in . When the Yankees moved their affiliation to the
Nashville Sounds, Merrill moved as well, guiding the team to two more first-place finishes in and . In and , Merrill managed the
Fort Lauderdale Yankees of the
Florida State League, winning the league championship in 1982. For the
1984 season, he jumped to the triple-A level, managing the
Columbus Clippers to another first-place finish. Merrill joined the New York Yankees in as first-base coach for manager
Yogi Berra, but was returned to Columbus in mid-season to again serve as that team's manager. He rejoined the major-league team in as a coach on
Lou Piniella's staff. During the season, Merrill was sent to the
Albany-Colonie Yankees, then the team's double-A affiliate in the Eastern League. He won the league title that season, and did so yet again in , guiding the
Prince William Cannons to the Class A
Carolina League championship. In , Merrill began the season with Columbus before getting a call to take charge of the New York Yankees in June when
Bucky Dent was fired as manager. Merrill finished out what would be one of the worst seasons in Yankees franchise history, which saw those Yankees finish at 67-95 (49-64 under Merrill), dead last in the American League and second-worst in all of baseball. It was only the fourth time the storied franchise had finished with the worst record in the league. However, Merrill was not blamed for the debacle and was brought back for the season—the first time in four years where the Yankees had a single manager for the entire season. However, after the Yankees could only improve to 71–91, he was replaced by
Buck Showalter prior to the
1992 season and served that season as a roving minor-league instructor. During and , Merrill once again managed the Columbus Clippers before spending as a special assignment
scout. In , Merrill guided the Clippers for a third time, winning the
International League title, the
Governors' Cup. Along the way, Merrill won his 1000th game as a manager on August 2, 1996. He remained with the Clippers for the and seasons before returning to New York. In and , Merrill had a two-year stint as special assistant to the general manager under
Brian Cashman. He returned to the dugout in to manage the Yankees' double-A farm club, then the
Norwich Navigators. When the Yankees changed affiliations after the
2002 season, Merrill moved with the team once more, managing the
Trenton Thunder in and . Merrill returned to New York in and was once again named special assistant to the general manager. He retired in 2014.
Managerial record ==Nickname==