Mr Sinclair was elected after receiving 185 votes, to 145 votes for sole challenger Gina Adcock.
The by-election was triggered after only four candidates stood for the five seats at the table in last year’s local government elections.
There were no informal or blank votes. The voter turnout was 30.22%, with 330 votes cast.
Gore District Council said Mataura born-and-bred Mr Sinclair was a keen fisherman, and a life member of the Mataura Angling Club.
Council chief executive Debbie Lascelles thanked both candidates for standing in the by-election and acknowledged the community members who took part in the vote.
"Community boards play an important role in ensuring local perspectives are heard in council decision-making," she said.
Mr Sinclair’s first outing as a newly elected representative will be at the board’s next meeting, on May 4.
