Smith began his hockey career in 1999 playing for his hometown Kingston Frontenacs of the Ontario Hockey League (OHL). That season, he played 15 games while backing up future NHL player Andrew Raycroft (currently with IF Björklöven).
After the start of the 2000–01 season, Smith moved to the Sudbury Wolves, where he played 43 games, registering a 2.52 goals against average (GAA) and 0.913 save percentage in the regular season. The Wolves made it to the second round of the playoffs that year with Smith in net.
In the 2001 NHL Entry Draft, Mike Smith was drafted by the Dallas Stars
with the first pick in the fifth round, and continued to play in
Sudbury, again taking the Wolves into the post-season. Their playoff run
ended in four games, however, as the Barrie Colts won the series 4–0.
The subsequent season saw Smith's departure from the junior leagues when he split the season between the Lexington Men O' War of the ECHL and the Utah Grizzlies of the American Hockey League (AHL).
Smith made his professional debut on October 26, 2002, against the Dayton Bombers,
scoring a statistically unlikely goal during a shutout in his first
win, scoring on an empty net at 19:04 in the third period. Smith is the
youngest goaltender ever to score in a professional game, doing so at 20
years of age.
He played only 11 games in Utah, backing up Jason Bacashihua and Corey Hirsch. For 2003–04, Utah added Dan Ellis to their roster, with whom Smith split back up duties, again to Bacashihua. Smith went to play for the Houston Aeros in 2004–05, sharing the net almost equally with Josh Harding, putting up a 0.915 save percentage and 2.42 GAA.
Smith joined the Iowa Stars for their inaugural season in 2005–06.
The Stars played Smith and Dan Ellis back-and-forth for much of the
season until Smith appeared to take much of the netminding
responsibilities late in the season. A Stars rally in the stretch
secured the final Western Conference playoff spot. Smith started in
every game of the seven-game series against the Milwaukee Admirals, which Milwaukee ultimately won.
Smith underwent shoulder surgery during the off-season for an injury
he sustained during Iowa's 2005–06 season. After a faster-than-expected
recovery, Smith attended the Dallas Stars' training camp in the fall of
2006, and was awarded the backup goaltender position behind Marty Turco. In his first-ever regulation NHL game on October 21, 2006, Smith stopped 22 shots to post a shutout against the Phoenix Coyotes.
On January 4, 2007, Smith was hit in the mask by a 100 mph slapshot, causing him to miss a few weeks with a concussion. He came back strong, however, winning two games in which Turco was pulled. He then registered a 32-save shutout over the Anaheim Ducks on February 10.
Smith also started the year living with Marty Turco's family.
However, he eventually moved out and then signed a two-year,
$950,000-a-year contract with the Stars.
On June 14, 2007, Smith was named to the 2006–07 NHL All-Rookie Team.
On February 26, 2008, Smith was traded, along with Jussi Jokinen, Jeff Halpern and a fourth round draft pick in 2009, to the Tampa Bay Lightning in exchange for Brad Richards and Johan Holmqvist. Smith made his Lightning debut the following night against the Minnesota Wild, stopping 24 of 27 shots in a 3–2 loss.
On March 15, Smith recorded a 3–0 shutout victory over the New York Rangers.
He became only the third rookie goaltender in NHL history to record a
shutout for two different teams in the same season, and the first to do
so in 79 years. Smith recorded 14 wins for the Lightning during the 2008–09 season before suffering a concussion
that kept him out of action for the remainder of the season. On
February 2, 2011, the Lightning placed Smith on waivers, where he
cleared the next day and was subsequently assigned to the Lightning's
AHL affiliate, the Norfolk Admirals. He returned to Tampa Bay later in the season, however, and, backing up Dwayne Roloson during the Lightning's 2011 playoff run, made his playoff debut in place of Roloson in the third period of Game 2 of the Eastern Conference Finals against the Boston Bruins.
On July 1, 2011, Smith signed a two-year contract worth $2 million with the Phoenix Coyotes.
Smith set the NHL record for the most saves in a regulation shutout
victory, as he stopped all 54 shots he faced in a 2–0 win over the Columbus Blue Jackets on April 3, 2012. However, his record was later surpassed on January 29, 2014, when Ben Scrivens stopped 59 shots in a 3–0 win for the Edmonton Oilers over the San Jose Sharks.
He went on to record an impressive 38 wins in his first season in
Phoenix, as the Coyotes qualified for their third consecutive playoff
berth, and finished fifth in voting for the Vezina Trophy, awarded to the league's top goaltender.
In the 2012 Stanley Cup playoffs, Smith led the Coyotes to their first playoff series win since 1987, beating the Chicago Blackhawks 4–2 in the series. Smith and the Coyotes went on to defeat the Nashville Predators in the Western conference semi-finals, winning the series 4–1. The Coyotes then faced the eventual Stanley Cup champions Los Angeles Kings in the Western Conference Finals, losing the series 4–1. He posted a superb 1.99 GAA and a .944 save percentage, some of the best numbers for starting goaltenders in the playoffs.
On July 5, 2013, Smith signed a six-year contract extension with the Coyotes worth $34 million.
On October 19, 2013, Smith became the seventh NHL goalie to score a
goal with a shot on goal (four other goaltenders have been credited
based on own-goals by the opposition). Smith scored with 0.1 seconds
left in regulation on an empty net during a win against the Detroit Red
Wings.
Two months later, Smith would score on himself when, as a result of the
hockey puck getting lodged into his waistband, he inadvertently backed
into his own net and scored an
own goal. The "butt goal" handed the game to the
Buffalo Sabres, as the game was in
overtime.
Smith won a gold medal with
Canada's
2014 Winter Olympic team, dressing as a backup for one of Canada's
group games. He did not see any ice time and was primarily the third
choice keeper. He also won gold at the
2015 world championships
as Canada's starting goalie, going 10-0 through the tournament and
recording a shutout streak of 190:03 in the medal round (the final three
games).