Sean Thornley played mainly on the ATP Challenger Tour and the ITF Future Tour and he was considered a doubles specialist. He celebrated 22 double victories on the Future Tour. On the Challenger Tour won the doubles tournament in Izmir in 2012 and the tournament in Tampere in 2014 . On October 17, 2011, he broke the top 200 in the world rankings in doubles and his highest ranking was a 160th place in September 2012.
Thornley twice qualified for a Grand Slam tournament. In 2011 and 2012 he played doubles at Wimbledon . Both times he lost the opening hurdle with his partner David Rice . In 2014 and 2015, he lost in the second qualifying round of the same tournament. In 2015 he played his last professional tournament.