Sinclair fired Town ahead in the 10th minute following a lightning fast counter attack in which Torquay striker Billy Bodin was injured as he teed up a strike at the visitors' goal from 20 yards.
Northampton broke from their own defensive lines at speed through the right channel with Sinclair clinically beating Gulls goalkeeper Michael Poke with an angled drive.
Sinclair doubled his tally in the 16th minute with an even better finish from an acute angle on the right.
Torquay substitute Jordan Chapell made a notable difference after replacing Bodin in the 21st minute and he forced Town keeper Matt Duke to turn a goal-bound 25 yard drive around the post five minutes before the break.
Chappell then took the corner from the left that central defender Krystian Pearce turned in at the near post with a superb side-foot volley.
Fellow central defender Anthony O'Connor hit the post with successive shots as Torquay pressed unsuccessfully for a second-half leveller.
- Sports & Recreation
- Billy Bodin