His father is Bobby Grier, who is the Associate Director of Pro Personnel for the Houston Texans of the National Football League (NFL). Prior to that, Bobby Grier was a running back coach, director of pro scouting, and vice-president of player personnel for the New England Patriots of the NFL. (The elder Grier should not be confused with the Bobby Grier who broke the color barrier in the 1956 Sugar Bowl.) Mike's brother is Chris Grier, General Manager of the Miami Dolphins. Another notable athlete in his family is Pro Bowl NFL Defensive Lineman Roosevelt "Rosey" Grier.
Grier was raised in Holliston, Massachusetts, where he attended and played hockey at St. Sebastian's School. At age 9 Michael appeared in Sports Illustrated's "Faces In The Crowd" section in issue 06-18-84/Vol. 60, No. 25, it read: Michael, 9, a defenseman for the 35-19-8 Holliston Mites in the 7-to-9 year-old division of the Westboro ice hockey league, had 114 goals and 43 assists this season. In 1982-1983 he racked up 113 goals and 104 assists in 52 games.
Grier was originally drafted by the St. Louis Blues in the ninth round (219th overall) of the 1993 NHL Entry Draft, and was considered a long-shot to make an NHL team. He spent his early playing days with Saint Sebastian's School and later with Boston University, culminating in his best amateur season in 1994–95, where he was named a first team all-star. During his time at BU, Grier's NHL rights were dealt to the Edmonton Oilers along with star goaltender Curtis Joseph in exchange for a pair of first round picks.
After leaving college, Grier immediately cracked the Oilers lineup as a checking-line right-winger, scoring 32 points and bearing a respectable +7 plus-minus rating. During his time in Edmonton, Grier was best known for provoking Chris Simon of the Washington Capitals in 1997. Grier allegedly made derogatory comments about Simon's Ojibwa heritage, and Simon allegedly responded with a racial slur directed at Grier, and struck him in the head with his stick. Although the spoken words were never confirmed, Simon was suspended three games as a result of using his stick to hit Grier. Grier played six seasons with the Oilers organization, including two in which he scored twenty goals.
On October 2, 2002, in order to free up roster space, Grier was traded to the Washington Capitals for a pair of draft choices. Incidentally, this put Grier and Simon on the same team for a short time; however, Simon was traded to Chicago after playing 10 games in the 2002–03 season. The Capitals attempted to put a Stanley Cup-caliber team together, primarily built around star forward Jaromír Jágr and goaltender Olaf Kölzig. The team disappointed on the ice, although Grier remained a reliable checking player.
This didn't last long, however, as Washington traded Grier on March 4, 2004 to the Buffalo Sabres for Czech prospect Jakub Klepiš. Grier finished the season with Buffalo, scoring nine points, but the Sabres failed to make the playoffs. During the 2005–06 NHL season, Grier set a personal record, scoring four game-winning goals for the Sabres and contributing to their run to the 2006 Eastern Conference Finals. Grier was then signed to a free agent deal by the San Jose Sharks, where he recorded 16 goals, including three shorthanded, in his first season with San Jose. He scored nine and 10 goals in his next two seasons with the Sharks, respectively.
On August 10, 2009, Grier returned to the Buffalo Sabres.
Grier's biggest contribution to the San Jose Sharks and Buffalo Sabres has been his penalty killing ability. San Jose ranked 14th, 1st, and 4th in penalty killing in the three years Grier played with the team. In the 2009–10 season, Buffalo was second overall in the NHL in penalty killing. Grier has been a part of the top penalty killing units for these teams.
Grier is also known for his toughness, separating his shoulder and having it popped back in on numerous occasions before finally electing for surgery.
Grier played his 1000th NHL game on November 3, 2010, against the Boston Bruins, becoming the 254th player in NHL history to reach the milestone. After not being re-signed by Buffalo for the 2011-12 season, Grier officially announced his retirement from the NHL on December 1, 2011.