2000 As a junior for the Exeter Swimming Club, he competed at the British Winter Championship in 2000 at the age of 15. He broke four records and won more medals than any junior under the age of 16 had before him.
2001 He followed this with a gold medal at the 2001 Youth Olympic Games.
2002 Two golds in 2002 at the World Schools Championships.
2005 He competed at a senior level for the first time in 2005 at the
2005 World Aquatic Championships, where he took the bronze medal in the 50m backstroke.
2007 At the Japan International Open in August 2007, he won two gold medals. The first was in the 100m backstroke, which broke the European record time, and his second was in the 200m individual medley with a time of 1:59.19, which was only the second time he had finished with a time of under two minutes. It was a new British record, beating the previous record by a second and a half.
2008 Tancock won several medals at the 2008 World Short Course Championships in Manchester, including a British, European and Commonwealth record time of 50:14 to take the gold medal in the 100m backstroke. The time was only 0.14 seconds off the world record set by American
Ryan Lochte. Competing at the
2008 Summer Olympics in
Beijing in the men's 100m backstroke, Tancock finished in sixth position with a time of 53.39, some 0.21 seconds behind the bronze medal position. Tancock said of the lack of a 50m event, "There is a 50m at every other major competition bar the Olympics. Don’t ask me why, but it never has been. It is not an issue but, of course, I would like it to be there".
2009 He broke his own world record winning the gold medal for 50m backstroke at the
2009 World Aquatics Championships. He had set a time of 24.08 in the semi-finals, but improved it with a time of 24.04 in the final. He wore a
bodyskin swimsuit which was subsequently banned by
FINA at the start of 2010.
2010 In the 50m backstroke event at the
2010 Commonwealth Games in Delhi, he took the gold medal once more, breaking the Commonwealth Games record with a time of 24.62 in the final. He also took a second Commonwealth gold in the 100m backstroke with a time of 53.59. and following his success at the Commonwealth Games he was named BBC South West's Sportsman of the Year 2010.
2011 At the
2011 World Aquatics Championships in Shanghai, Tancock retained his world championship crown with a time of 24.5 seconds in the 50m backstroke. It was the second occasion a British male swimmer had retained a world championship title, and he became the first man to retain the 50m backstroke title. Whilst competing at the venue, which was the swimming venue at the 2012 Games, he did not find the controversial ceiling and lighting set up distracting, putting it down to the training he conducted for the 2009 World Championships, which were held outside. As part of his training regime for the Games, he took up
ballet,
kickboxing and rock climbing in order to improve on his position of sixth at the 2008 Games. He was also part of the Great Britain team that finished fourth in the 4 × 100 m medley relay, where they finished 32 milliseconds behind the Australian team in third. In November 2020, it was reported that Australia's
Brenton Rickard tested positive for a banned substance which could lead to the GB Team being awarded a retrospective bronze medal.
2013 Tancock achieved the World Championships qualifying time, but the team coach only chose swimmers who had a chance of being at Rio 2016. As a result of the 50m backstroke not being an event at the Olympics, Tancock was not selected for the team.
2014 At the 2014 Commonwealth Games, Tancock won bronze medals in the men's 50 metre backstroke and the 100 metre backstroke and helped England to win the 4 x 100 metre medley.
2015 At the age of 30, Tancock qualified for the final of the
100m backstroke at the
World Aquatics Championships in a time of 53.19. ==Personal bests and records held==