NSW Academy program has churned out many players and Henry Thornton is one of those umpteen players that the NSW Academy feeds to the New South Wales state side. Henry joined the academy as an under-13 player and moved up the ranks to play for Australia under-19s against England's colts in 2015. He picked up 19 wickets in 5 Youth ODIs and one four day game to emerge as one of the success stories of the tour.

His state side NSW rewarded him a development rookie contract and so did Sydney Sixers for BBL for the 2015-16 season. In the following season the Thunder unit also fielded him in a game against Brisbane Heat in early 2017. He conceded 40 runs off 4 overs in a high scoring game picking up one wicket. The Sixers ended up winning the game. The year 2017 also saw him making his list-A debut as he was picked for the Prime Minister's XI to square up against the touring Pakistanis.

Later the same year he was part of the CA XI for the JLT One-Day cup and featured in three games. Thornton again took a few steps forward and got a full contract with both NSW and the Sixers. An injury just before BBL 07 did not help his cause though.