A product of Mooroolbark, in Melbourne's outer eastern suburbs, Michell played in the under 18 TAC Cup competition with the Eastern Ranges. He was the club's Best and Fairest in 1999 and 2000. Disappointed at being overlooked in the 2000 draft Mitchell joined the Box Hill Hawks. After a couple of games in the Development squad he gained promotion to the seniors and completed the season as the team's number one rover. He was a member of Box Hill's premiership side in 2001.
The first half of his debut season in 2002 saw him playing with the Box Hill Hawks, until he broke into the Hawthorn side midway through the season. Following some unimpressive performances where he never managed more than 14 disposals, he was dropped for Round 15 but was recalled after more eye-catching performances in Round 19. He polled 31 votes in just 11 games to win the VFL's best and fairest award, the J. J. Liston Trophy.
In 2003 Mitchell continued to improve, winning the 2003 AFL Rising Star award and becoming known as "the Extractor" (particularly by commentator Brian Taylor) for his high amount of clearances and ability to win the ball out of middle of the ground.
A solid season followed in 2004, and in 2005 he played a "super" season until a foot injury sidelined him in Round 15. For the 2006 season, Mitchell was named vice-captain of Hawthorn and displayed stellar form throughout the season culminating in winning the Peter Crimmins Medal for Hawthorn's best and fairest.
In 2007 he capped off another wonderful season by coming 3rd in the votes for the highest honour in Australian Football, the Brownlow Medal with 21 votes, 1 short of joint 2nd-place winners, North Melbourne's Brent Harvey and Brisbane's Simon Black who both polled 22 votes, and coming 8 votes behind winner of the 2007 Brownlow, Geelong's Jimmy Bartel.
On 6 October 2007, during the Peter Crimmins Medal Event, he was announced as Hawthorn's next captain, taking over the reins from retiring captain Richie Vandenberg.
Mitchell was ineligible for 2008's Brownlow Medal following a tripping charge in the match against Melbourne in round nine.
On Saturday, 27 September, Mitchell captained the Hawks to the 2008 Premiership, the first in 17 years and the clubs 10th, beating the reigning premiers, Geelong, by 26 points. Mitchell was reported for rough conduct against Geelong superstar Gary Ablett, Jr. in the second quarter, however this report was dismissed at the conclusion of the weekend.
Mitchell polled 30 votes in the 2011 Brownlow Medal, but was ineligible to win after an incident in Round 5 of the season; he had the second-most votes, behind Collingwood's Dane Swan. In 2012 Mitchell was runner-up in the Brownlow to Jobe Watson.
Sam Mitchell has won the Peter Crimmins Medal four times (2006, 2009, 2011, 2012), the most of any current Hawthorn player.
Mitchell is often cited as one of the most ambidextrous player in the AFL and much opinion is made about which is his preferred foot. Although some have claimed that he was originally a left footer who switched to right foot later in his junior career, Mitchell has stated that he has just always tried to use the appropriate foot for the situation. He does however switch to his left foot when he wants to kick longer. Similarly, his handpassing is equally good with either hand.
Another of Mitchell's superior abilities is his ability to change direction quickly. He has himself joked about his short legs and lack of leg speed, however his ability to side-step through congestion and confuse the opposition is considered amongst the best in the competition. This, paired with the opposition not being able to predict which foot or hand he will pass with makes him a difficult player to corral.
On 13 July 2015 Sam Mitchell was found guilty by the AFL's Match review Panel of having kneed the right thigh of Fremantle Docker Nathan Fyfe during the second quarter of the previous day's game and was fined $1000 for the offence. The media then brought to light other kneeing incidents involving Mitchell, those being the kneeing of Adelaide Crows captain Taylor Walker in the round 12 game and the kneeing of Greater Western Sydney's Ryan Griffen in round 6 of the 2015 season. Another kneeing video incident surfaced of Mitchell kneeing North Melbourne defender Scott Thompson. The video shows Mitchell walking up to an unsuspecting Thompson and kneeing him in the left thigh. Media reports from 2008 show that an opposition club, Brisbane expressed concerns to the AFL about Sam Mitchell's kneeing of opponents.