Born in Katherine, Northern Territory, Australia, Sinclair moved to New Zealand with his mother after the death of his father in an accident when he was only five years old.

A right-handed middle-order batsman occasionally used as an opener, he played for Central Districts from the 1995–96 season, and in a period when the New Zealand cricket authorities were actively developing their infrastructure with the aim of raising the standards of the New Zealand team, he played for both Academy and A teams before making his Test debut.

He scored 214 on his debut, against West Indies at Wellington in 1999, and followed that with 204 not out against Pakistan in the following summer. But despite this most promising of starts, Sinclair struggled to gain a permanent place in both the Test and one-day sides after disappointing scores. As a result, he was intermittently featured in the international squad, most recently being the One Day International against the West Indies at Eden Park in January 2009.

An injury to Michael Papps in 2004/05 saw him called into the tour of Bangladesh as a 'makeshift' opener. His preferred position in the top of the middle-order was not available at the time. He did enough on that tour to gain selection for the tour to Australia where he had mixed results in the Tests, but not enough to maintain his spot when the Australians crossed the Tasman later that summer.

His form in the ODIs in Australia was sufficient to retain his place for the return series, but he lost his place after averaging 15 in the first three matches.

In July 2013 he announced his retirement from all cricket. At 37, after 18 seasons in the Central Districts side he is the team's all-time highest run-scorer, with more than 15,000 runs across all formats and remained a solid performer to the last, averaging over 40 in 2012-13. It was also revealed that he was unemployed.

Test centuries