Singletary attended college at Baylor University in Waco, Texas. As a linebacker at Baylor, Singletary earned All-America honors in both junior (1979) and senior (1980) years, where he averaged 15 tackles per game and established a team record with 232 tackles in 1978, including 35 in a game against the University of Houston. During Singletary’s senior season of 1980, Baylor won 10 games, marking the first time in school history that had been accomplished.

Singletary is the only college junior to be selected to the All-Southwest Conference Team of the 1970s. Singletary is a two-time recipient of the Davey O’Brien Memorial Trophy, which at the time was awarded to the most outstanding player among those playing in the southwestern United States and had yet to become the quarterback-centric award it is today. Singletary lettered four years. He had 97 tackles as a freshman, 232 (a school record) as a sophomore, then 188 and 145. The total, 662, set a school record. In 1978, he had 35 tackles in a game against Arkansas, 31 against Ohio State. He was all- Southwest Conference three years and All-America two years. In 1979 and 1980, he won the Davey O’Brien Award, given to the outstanding player in the Southwest. (The award has since been changed to a national quarterback trophy.)

He led the Bears to a 15-1 season in 1985. The “46” defense invented by Buddy Ryan allowed Singletary to be unblocked on virtually every play. That season, he recorded 109 solo tackles (52 assists), 3 sacks, 1 interception, 3 fumble recoveries, 1 forced fumble, and 10 defended passes. He won the NFL Defensive Player of the Year, and led a defense that was ranked 1st overall (1st against the rush; 3rd against the pass).

Throughout the 1985 playoffs, Singletary provided stellar efforts in all 3 games. In the divisional game against the New York Giants at home, Singletary recovered a fumble early in the 1st quarter and a sack of quarterback Phil Simms on a 3rd down early in the 3rd quarter. The Bears went on to win 21–0. In the NFC Championship Game vs. the Los Angeles Rams, Singletary and the Bears dominated again. Coach Mike Ditka said that the day before the game, he was talking to the offense while Singletary was in the next room giving the defense a motivational speech. While it started out quiet, within minutes, Samurai Mike was screaming at the top of his lungs and the defensive players were throwing chairs and knocking over tables. The Bears would eventually go to win Super Bowl XX by beating the New England Patriots 46–10. In the game, Singletary broke up a pass that would have gone for a touchdown, delivered jarring hits to New England running back Craig James all game, and tied a Super Bowl record with 2 fumble recoveries.

Singletary was named NFL Defensive Player of the Year by the Associated Press in 1985 and 1988. He was elected to the College Football Hall of Fame in 1995 and to the Pro Football Hall of Fame in 1998. In 1999, he was ranked number 56 on The Sporting News’ list of the 100 Greatest Football Players.

At 6 ft (1.8 m), 230 lb (100 kg), Singletary became a starting linebacker in the Chicago Bears defense in the seventh game of his rookie season (1981). In a game against the Kansas City Chiefs, his third as a starter, Singletary gave a remarkable defensive performance, recording 10 tackles and forcing a fumble. He was a nearly unanimous all-rookie selection. Singletary started 172 games for the Bears during his 12-year career, the second most in club history. An intense player, he finished as the Bears’ first or second leading tackler each of his last 11 seasons. He amassed an impressive 1,488 career tackles, 885 of which were solo efforts. He missed playing just two games, both in 1986. He made 7 interceptions and 12 fumble recoveries.

In a game against the Denver Broncos in 1990 he had a personal-best performance when he recorded 10 solo tackles and 10 assists. Selected to play in a team record 10 Pro Bowls, Singletary was All-Pro eight times, and All-NFC every year from 1983 to 1991.

In 2002, a group of alumni at Baylor University promoted Singletary for the school’s open head coaching position, holding a conference call (which included Mike Ditka and Dave McGinnis) with the university administration. The position ultimately went to Guy Morriss. Singletary at the time had no head or assistant coaching experience and the university president later stated that they wanted to hire a coach with prior “head coaching experience.”

In 2003, Singletary became the linebackers coach for the Baltimore Ravens.

Following the 2004 season, the San Francisco 49ers hired Ravens defensive coordinator Mike Nolan to be their head coach, and Singletary left the Ravens with Nolan to be San Francisco’s assistant head coach and linebackers coach. In 2007, Singletary interviewed for the head coaching job of the Dallas Cowboys and San Diego Chargers, but was ultimately passed over.

After being out of the league since 2014, Singletary interviewed with the New York Giants for their vacant linebacking coach position. Giants head coach Ben McAdoo and Singletary both worked together with 49ers in 2005. The Giants eventually hired former Philadelphia Eagles assistant Bill McGovern for the position.

With Morriss leaving Baylor after the 2007 season, Singletary appeared to be the leading candidate for his replacement, and expressed interest in the job. On November 19, 2007, Baylor athletic director Ian McCaw flew to San Francisco to interview Singletary. However, Singletary decided against taking the position, which instead went to Art Briles. Singletary’s son Matt was a freshman on the Baylor team in 2007.

Singletary and his wife, Kim, have seven children. His son Matt joined Baylor University’s roster in 2007 as a freshman defensive end. However, Matt transferred to California Polytechnic State University where he was a junior defensive end. Singletary’s nephew, Vantz, is a linebackers coach at Liberty University and previously coached under Mike when he coached with the 49ers.

Singletary became the interim head coach of the 49ers after Nolan was fired on October 20, 2008. Singletary instantly made a statement in San Francisco by sending tight end Vernon Davis to the locker room with more than 10 minutes remaining in his head coaching debut, a 34–13 loss to the Seattle Seahawks on October 26, 2008. Davis was penalized 15 yards for slapping Seattle safety Brian Russell after a reception. In a post-game press conference, Singletary called Davis’ actions “uncoachable” and said that he would rather play with a 10-man squad than have to deal with an apathetic 11-man squad. It was later reported that during halftime of their game against the Seahawks, Singletary intentionally dropped his pants while giving a speech to illustrate just how poorly the 49ers had played in the first half.

Taking over the 2–5 49ers team, Singletary was able to finish the season 5–4 under his leadership (for a final team record of 7–9). On December 28, 2008, after a 27–24 come-from-behind victory over the Washington Redskins, Singletary was offered the team’s long-term head coaching position, and signed a four-year, $10 million contract as the 49ers’ head coach.

After leading the 49ers to a positive finish in 2008, Singletary helped the team jump out to a 3–1 overall start in the 2009 season. Despite a last-second loss to the Brett Favre-led Minnesota Vikings in Week 3, the 49ers recorded wins against the reigning NFC champion Arizona Cardinals and NFC West divisional rival Seattle Seahawks and St. Louis Rams.

Five players on the 2009 team were named to play in the NFL Pro Bowl. Those players were Patrick Willis, Andy Lee, Vernon Davis, Frank Gore, and Justin Smith.

Also in 2009, Singletary joined other members of the 1985 Chicago Bears in resurrecting the Super Bowl Shuffle for a Boost Mobile commercial broadcast during Super Bowl XLIV.

On January 18, 2011, Singletary confirmed that he had agreed to join the Minnesota Vikings coaching staff as the linebackers coach/assistant to the head coach.

On June 28, 2016, Singletary in an interview with a radio station he announced he will be returning to the coaching ranks as an advisor for the Los Angeles Rams defense. The move reunited Singletary with former Bears teammate and then-Rams head coach Jeff Fisher.

On May 10, 2018, Singletary was named head coach of the Memphis Express of the Alliance of American Football. The Express went 2–6 before the league cancelled the rest of the season. The league eventually ceased operations altogether.

On March 29, 2018, Singletary accepted a job at Trinity Christian Academy in Addison, Texas. He left the school in December 2019 after going 1–21 in two seasons.