In 1977, Sinclair made his first grade debut with South Sydney off the bench in round 7.[1] He started the following game against Cronulla-Sutherland at half-back.[2] Two rounds later, he scored the first try of his career in a round 10 win over Newtown.[3] In round 12, he scored a try in a 13-17 loss to Balmain.[4] Sinclair scored in the final round of the season against Newtown, however Souths lost the game 20-33 and received their 11th straight loss.[5]
Sinclair made 6 appearances in the 1978 season and did not play in 1979.
In 1980, Sinclair returned with South Sydney off the bench in round 5 against Newtown. He would not experience success until later in the season, when he scored a try in three consecutive games from Round 20 to Round 22.[6][7][8] He played in the minor preliminary semi final, though South Sydney lost to the St. George Dragons.[9]
Sinclair played 14 games in 1981. In Round 13, he scored the final try of his career in a win against St. George.[10] Sinclair played his final NSWRL game in round 16 in a 10-13 loss to Balmain.[11] He concluded his career with 7 tries from 53 games.