Those returns earned him a development rookie deal with New South Wales, and the Sydney Sixers handed him a similar arrangement for the 2015-16 Big Bash League season. His first BBL appearance came in early 2017 in a high-scoring match against the Brisbane Heat, where he bowled 4 overs, gave away 40 runs and took one wicket. That same year he made his List A debut, chosen for the Prime Minister's XI to face the visiting Pakistan side.
Later in 2017 Thornton turned out for the CA XI in the JLT One-Day Cup, playing three matches. His progress was then recognised with full contracts at both New South Wales and the Sixers. However, an injury shortly before BBL 07 set him back, and his next appearance in the competition did not come until 2021/22, when he lined up for the Adelaide Strikers.
Thornton's domestic career has also taken him beyond New South Wales. He spent the 2021/22 season with Victoria and then relocated to South Australia for 2022/23.