It was inevitable he was going to either stay on the farm or choose a career in a related field, and he saw veterinary science as a good option.
Mr Sinclair, who was recently elected president of the New Zealand Veterinary Association (NZVA), has no regrets about pursuing that path.
Since graduating in 1979, he has spent nearly 30 years working in Gore, dealing mostly with deer, sheep and beef cattle.
He is a director of VetSouth Ltd and works from the company's Gore practice, which has 13 vets. There are also practices in Invercargill, Otautau, Winton, Tapanui and Lumsden.
Mr Sinclair said he enjoyed the contact with farmer clients and the interaction with colleagues, not just at work but also around the country.
He has seen many changes in the rural sector since first moving to Gore. Back in 1981, there were seven dairy farms, which were all town supply.
Not only had that number swelled to more than 100, but the herd size had also considerably increased.
Sheep productivity had gone up "hugely" and deer farming had gone from being a hobby activity to a major endeavour.
With farmers enjoying good prices, it was a "real buzz" to have every sector of the farming community happy at once, which he had not seen in 30 years.
There was much more awareness about animal welfare and looking after the environment for future generations.
Obviously, incidents such as the severe storm in the South last September, in which a million lambs died, were very difficult for farmers.
But there was an awareness that farmers had to do better with shelter, particularly for sheep in the South.
Coming from a different professional background from his predecessor at NZVA, Dr Richard Wild, Mr Sinclair will bring different attributes to the role of president.
NZVA is the membership organisation for veterinarians in New Zealand. About 75% of all registered vets are members, which Mr Sinclair said was high by international standards.
A lot of registered vets in New Zealand did not work directly in practices and the organisation was trying to encourage membership across all branches of the profession, he said.
NZVA had been heavily involved in redrawing the Code of Professional Conduct for Veterinarians, which will be finalised later this month.
He saw his role, which was around the governance of the association, as a good challenge and he was looking forward to it.
It was about half a full-time role, so he still hoped to have sufficient time for his clients.
Challenges faced by the profession included the supply of vets in rural areas - getting them there and keeping them there was an issue faced by veterinary associations around the world, he said.
The vet bonding scheme, which the NZVA advocated several years ago, had been a "great help".
There were also challenges around animal welfare, biosecurity and quality assurance. Another issue was looking at companion animals, who were a big part of people's lives.
Just as with people, there was an "obesity epidemic" among companion animals, along with issues around inherited disorders.
It was the role of vets to maintain quality of life of those companion animals, he said.