Sinclair was first elected to Etobicoke Council as alderman for Ward 3 in 1969, defeating incumbent Alec McNeil in a recount.[3] In the 1972 election, he was elected to the borough's Board of Control, which also gave him a seat on Metro Toronto Council.[4] He remained on the Board of Control until 1984 when he was chosen mayor by Etobicoke council, to fill a vacancy caused when incumbent mayor Dennis Flynn stepped down upon being elected Metro Toronto Chairman.[1]
Before public office, Sinclair worked for Bell Gouinlock (Securities) Limited and Etobicoke Board of Education (as a math teacher, counsellor and consultant).[5] In his first few months as mayor, Sinclair continued to operate a consulting business he had run for several years, giving advice to Ontario municipalities on debenture financing, leading to criticism from other members of council.[6]
Sinclair died on March 24, 2023, at the age of 94.[7]