Born in Hobart, Tasmania, Riewoldt and his family moved to the Gold Coast, Queensland, when he was 9 years old. He attended Robina State Primary School from Grade 5 and became the school's vice captain and UNICEF representative. He later switched to All Saints Anglican School during his high school years. Australian rules football was not offered in many Queensland schools at the time but he made a name for himself as an overall sportsman, participating in the school's athletics, cross country, cricket, soccer and touch football teams at a high level. Academically, he also excelled in the studies of physics and chemistry.
Riewoldt played his junior and early senior football for the Broadbeach Cats before moving to the Southport Sharks where he was identified as an outstanding junior talent. He played a big part in Southport's historic fourth consecutive QAFL premiership in 2000. After being kept virtually touchless for the first half, he was moved into the ruck and kicked two important goals in the 11-point grand final victory.
His breakout season was 2004 when he took an AFL record 256 marks and also finished in the top ten for both the Coleman and Brownlow medals. He received the AFL Players Association Most Valuable Player Award, the Leigh Matthews Trophy and was selected in the 2004 All-Australian Team for the first time.
Riewoldt was rewarded for his 2004 efforts when St Kilda named him club captain for 2005 as part of their rotating captaincy policy. His 2005 season began badly as he broke his collarbone in the third quarter of the first game of the season against the Brisbane Lions. This drew controversy as he was then bumped after the injury by Lions players Chris Scott and Mal Michael when it was obvious he was hurt. Riewoldt had, however, waved away the trainers indicating that he was fine to continue playing, so no action was taken against Scott or Michael.
After a brief return to playing later in the season, Riewoldt again broke his collarbone.
Early in the 2006 season, Riewoldt returned to close to his best form, kicking five goals against former powerhouse Brisbane in Round 3. This match seemed to have put Riewoldt's kicking "yips" to bed, but they returned during the Round 7 match against Geelong when he missed a simple but critical goal from 15 metres out. This miss was forgotten the next week when Riewoldt kicked 9 goals against Carlton and took many contested marks. Riewoldt also took a match saving mark for the Saints against Sydney in Round 11, a match the Saints won in wet conditions, and one that kept their finals hopes alive.
Riewoldt started the season well, with eight goals in his first two matches.
In Round 7, Riewoldt dominated against the Sydney Swans, which kick started another outstanding year. Despite the Saints missing the finals, Riewoldt enjoyed a better year statistically than 2004, narrowly winning St Kilda's best and fairest Trevor Barker Award ahead of defender Sam Fisher.
Riewoldt played in St Kilda’s 2008 NAB Cup winning side – the club's third pre-season cup win.
Riewoldt had a consistent 2008 season, coming second in the AFL for marks (behind Matthew Richardson) and kicking 57 home and away season goals (63 all up) to finish ninth in the competition. This performance earned him his third All-Australian selection, his first at centre-half-forward. He was also selected to play in the Hall of Fame Tribute Match, but was ultimately unable to play due to a knee injury.
Riewoldt was recognised for his season with selection in the 2008 All-Australian Team as a forward player. This was his third All-Australian Team award.
Riewoldt took the most marks of any player in the league, averaging 9.7 a game and kicking 65 goals in the home and away season, averaging 3.3 a game.
He was a significant player in a St Kilda team which went through the home and away season with 20 wins and 2 defeats.
Riewoldt played in 21 of 22 matches in the 2009 home and away rounds in which St Kilda qualified in first position for the finals series, winning the club’s third minor premiership.
Riewoldt was recognised for his season with selection in the 2009 All-Australian Team as a forward player. He was also named captain of the 2009 All-Australian Team, his fourth All-Australian Team award.
In the preliminary final against the Western Bulldogs, Riewoldt kicked four goals, one of which was in the final minutes..
St Kilda qualified for the 2009 AFL Grand Final after qualification and preliminary finals wins but were defeated by Geelong by 12 points. Riewoldt kicked one goal.
Riewoldt won the 2009 Trevor Barker Award for St Kilda's best and fairest player, setting a new club record of five career best and fairest awards. He was also St Kilda's leading goal kicker in 2009.
During St Kilda's 28 point Round 3 win over Collingwoood, on 9 April 2010, Riewoldt experienced a high grade hamstring tear. He underwent surgery and made his return in Round 15, playing 15 games in the season for 39 goals. Riewoldt has now passed 400 goals in his career and is the 4th highest goalkicker in St Kilda's history.
On 20 December 2010 a photo with a nude image of Riewoldt was posted on the social networking site Facebook by a 17-year-old girl. The girl, whose name was initially withheld in the media due to her age, stated that her actions were a result of unfair treatment at the hands of the AFL regarding an incident earlier in the year, in which she claimed she had become pregnant as a result of having sex with two St Kilda players following a school football clinic. Another nude photo, of fellow Saints player Nick Dal Santo, was also posted. In a subsequent press conference Riewoldt claimed to have never met the girl and that the photo had been taken over a year prior, during an end-of-season trip, in Miami, Florida, by teammate Sam Gilbert, and that he had asked Gilbert to immediately delete the photo. Gilbert stated that the photos had been stolen from his own personal computer, describing the incident as a "significant breach of privacy without my knowledge or consent". The girl disputed Gilbert's statement, saying that she had taken the photos and was in possession of at least 19 more images of other AFL players. The girl later admitted this to be a lie, stating that she fabricated this claim of ownership so as not to get into any trouble. She then claimed to have been sent the photos by an unnamed St Kilda player in April of that year. On 24 December 2010 a Federal Court granted a permanent injunction on the images, supporting Riewoldt and Gilbert's statement that the photos had been stolen from Gilbert's laptop computer without consent. Justice Shane Marshall ordered the images and any others be deleted from the girl's computer, with mediation between her and the St Kilda Football Club scheduled to take place on 28 January 2011.