sinclair was elected as a liberal to caithness county council in 1949. in 1957 he was elected to thurso town council. he served on both the county and town councils until 1961 and then again from 1965 until 1973. he was a member of the scottish liberal party executive. he was liberal candidate for the east aberdeenshire division at the 1966 general election. the constituency had been safe conservative in 1964 with the liberals second. he managed to increase the liberal vote and achieve a swing of 5.5%, however, the conservative held onto the seat. he did not stand for parliament again.