[1] He took further lectures in medicine and in 1835 joined HMS Sulphur on a surveying expedition to the South American coast, under the command of Captain Frederick William Beechey, and afterwards of Sir Edward Belcher.

Sinclair collected plants in California, Mexico, Central America and Brazil, which he continued to send to the British Museum or the Kew Gardens.

[1][2] When Sinclair regained his health, he started work as a surgeon on convict ships to Australia and had opportunities of collecting at several Australian ports.

He joined missionary William Colenso (himself an avid botanist) and Joseph Dalton Hooker (assistant surgeon for Ross and one of the founders of geographical botany) on several botanical expeditions in the Bay of Islands.

FitzRoy immediately summarily dismissed Willoughby Shortland, who as Colonial Secretary had been acting Governor since William Hobson's death in September 1842.

[3][4] FitzRoy did not want to appoint anybody connected to any political faction to the position of Colonial Secretary and urged Sinclair to take on that role, who at first declined quoting his inexperience in administrative matters, but eventually agreed to save the Governor from an embarrassing situation.

His post ended after twelve years when New Zealand obtained responsible government[1] and the first Premier Henry Sewell took on the role of Colonial Secretary himself on 7 May 1856.

He made arrangements with Sir Julius von Haast to explore Mount Cook, but was drowned on 26 March 1861 endeavouring to cross the flooded Rangitata River.

[1] His zoological specimens, chiefly sponges and zoophytes, were mostly presented to the British Museum, and his plants to Sir William Hooker, who commemorated him in the tropical American genus of Asteraceae, Sinclairia.

16 New Zealand plants were named to commemorate Sinclair[1] including Meryta sinclairii, which is a popular street tree in coastal areas.