McCormack played college football at Kansas and assumed that he would take up a career as a high school coach. He was drafted by the New York Yanks in the 1951 NFL Draft, but had to wait until the third round before being taken. After the 1951 season concluded, he was conscripted into the US Army and served in the Korean War. While he was away, the Yanks moved to Dallas and became the Dallas Texans, which folded after just one season. McCormack came home in 1954 to find that his team had ceased to exist, so he became a free agent and was immediately signed by the Baltimore Colts, a new franchise created the previous year to replace the defunct Yanks/Texans. Cleveland Browns founder Paul Brown had not forgotten seeing McCormack play in his rookie season three years earlier and was sufficiently impressed that he decided to add him to the roster in a trade exchange with Baltimore. In his first season with the team, he played on the defensive line, and famously grabbed the ball out of Lions QB Bobby Layne's hands (in what the referees ruled as a fumble recovery) in the 1954 NFL Championship game against the Detroit Lions helping set up an important early touchdown.
The following season, he was shifted to offensive tackle and helped the Browns once again capture the NFL title. He would play a key role in helping legendary running back Jim Brown become one of the dominant players in the game, ending his career with four selections to the Pro Bowl.
Paul Brown. legendary Cleveland Browns founder, owner, and coach, stated in his 1979 memoir, PB: The Paul Brown Story, "I consider (Mike) McCormack the finest offensive tackle who ever played pro football." Also, according to Paul Zimmerman's 1984 book, The New Thinking Man's Guide to Pro Football, Brown also stated that McCormack was the best offensive lineman he ever coached. The book states that McCormack "[c]ould handle the Colts' Gino Marchetti better than any tackle in the game. Power combined with great intelligence and 4.8 speed. 'I've seen him have games,' former player and NFL executive Bucko Kilroy says, 'where if you were grading him, he'd score 100. Not one mistake, and his guy would never make a tackle.'"
McCormack retired from playing in 1962 and began coaching with the first of four consecutive stints as an assistant in the annual College All-Star Game. In 1965, he was hired as an assistant coach with the Washington Redskins, spending the next eight seasons working under four different head coaches, including former teammate Otto Graham from 1966–1968.
In 1973, he was hired as head coach of the Philadelphia Eagles, but got fired after the 1975 season and a record of 16-25-1. He moved on to serve as an assistant with the Cincinnati Bengals from 1976–1979, then was hired as head coach of the Baltimore Colts in 1980. He proved no more successful as a head coach in Baltimore than he had been in Philadelphia and was released after two seasons and a 9-20 record. As McCormack put it, "I wanted to be like my mentor, Paul Brown. He was a great teacher and I tried to do the same but unfortunately I always let my emotions carry me away."
In 1982, McCormack joined the Seattle Seahawks, eventually becoming president and general manager. He also served as the Seahawks' interim head coach for the remainder of the 1982 season when Jack Patera was fired after the first two games. McCormack took over during the 57-day players strike and led the team to a 4-3 record, the only time he compiled a winning record as an NFL head coach. He then returned to his management position when the Seahawks hired Chuck Knox as their new head coach in 1983 and declined all further offers to become a head coach.
In January 1989, he was abruptly fired by the new Seahawks owner, Ken Behring, who explained the decision was necessary in order to make changes in the financial operations of the team. Later that year, McCormack became a consultant for Jerry Richardson and his ownership group that were seeking to land an NFL expansion team in Charlotte, North Carolina. In 1993, he was hired by the newly formed Carolina Panthers as their team president and general manager. He retired from the Panthers organization in 1997, which erected a monument in their stadium honoring him.
McCormack died of heart failure on November 15, 2013 in Palm Desert, California.