Ryall played junior football for West Pymble Soccer Club in Sydney. Ryall later played for the AIS in the Victorian Premier League before signing for Melbourne Victory on a 2-year contract in August 2007.
He made his first appearance for the club in their round 3 match against Adelaide United and played in their Grand Final win over Adelaide in March 2009. In April 2009 he signed with Sydney FC for the 2009/2010 A-League campaign.
He made his first start for Sydney FC after coming back from his suspension in their Round 5, 2-1 loss to Gold Coast United.
He re-signed with Sydney FC for a further 2 seasons, on 24 August 2010.
Ryall scored his first goal for Sydney FC in the 2011-12 A-League season with a dramatic equaliser in the 90th minute, against his former team Melbourne Victory on 26 January. Ryall's strike came less than a minute after being brought on as a substitute . Ryall continued to score important goals for Sydney against his former club, including 3 goals in 4 appearances against the Victory during the 2013-14 A-League season with goals in the Round 5, 3-2 win at Allianz Stadium, Round 16, 5-0 win on Australia Day at Etihad Stadium in Melbourne, and again in the 1-2 2013-14 Elimination Final loss against the Victory at Etihad.
Controversy surrounded Ryall during the 2014-15 A-League round 17 clash vs the Melbourne Victory when he was awarded a penalty following a possession contest with Victory player Gui Finkler. Initial replays appeared to show Ryall simulating contact with Finkler's leg. At the time, Melbourne Victory were leading 2-1, and the resulting conversion of the penalty kick allowed Sydney FC to get back into the game. The decision to award the penalty was heavily criticized by media commentators, managers, players, and pundits alike, with former player and Fox Sports commentator Mark Bosnich reacting angrily to the decision. Melbourne Victory player Kosta Barbarouses claimed he had "lost respect" for Ryall over the incident. Ryall also drew criticism from Sydney FC coach Graham Arnold, and CEO of the club Tony Pignata over his reaction following the awarding of the penalty where he appeared to clip Finkler over his head. On 19 February, Ryall was cleared of diving by the FFA Match Review Panal, with the board taking 2 hours to clear him following reviewing slowed down footage, which showed Ryall's leg being clipped by Finkler.
Ryall signed a 2-year contract extension along with team mate Rhyan Grant midway through the 2014-15 A-League season, which will keep him at the club until the end of 2017.
Throughout 2007 and 2008 Ryall was the captain of the Australian U-20 side. Due to a suspension, Sebastian missed out on selection for the 2009 FIFA U-20 World Cup due to his criminal charges, where which he would have been assured a spot in the team. After a lengthy absence from Australia's national teams, in September 2010 he was called up to the Australian under-23 side by coach Aurelio Vidmar for a Four Nations Tournament held in Vietnam.
Ryall was charged, on 15 May 2009, with engaging in a sexual act with a 13-year-old girl. The alleged incident happened in January 2008, while he was playing with Melbourne Victory. He was suspended from participation in football matches in Australia, until 3 September as well as being stood down from the national youth team by Football Federation Australia Chief Executive Officer Ben Buckley for bringing the game into disrepute. On Monday 25 January 2010, he was committed to stand trial on the case after his committal hearing at Sydney's Downing Centre Local Court On 15 May 2010, the Director of Public Prosecutions dropped the charges against Ryall After a submission by Ryall's legal team that at the time of the alleged incident, he believed the girl to be over the legal age of 16 and that they were "just kissing" As of 7 September 2015
With Australia:AFF U19 Youth Championship: 2008
Weifang Cup (U-18): 2007
With Sydney FC: A-League Premiership: 2009-2010
A-League Championship: 2009-2010
With Melbourne Victory: A-League Premiership: 2008-2009
A-League Championship: 2008-2009