Thornely started his first-class career with Sussex in 2007 season, after scoring 800 runs in the Second XI Championship.
[1] In the 2011 season he played 2nd XI cricket for Somerset, Nottinghamshire, Essex and Kent, but did not win a contract at any.
Although, he did play first-class cricket for the Unicorns, a team made up from players without county contracts.
Whilst playing for the Unicorns he scored an unbeaten century against Somerset.
In 2012 he had a trial with Leicestershire and impressed enough to earn a contract and went on to score centuries against Glamorgan on his debut, and later against Essex.