Former local commissioner Richard Ronan (pictured) topped the poll by a relatively small margin after a recount in Castletown.
Mr Ronan, who resigned from the local authority in February, frustrated by its lack of power, polled 520 votes, beating teacher Jason Moorhouse by 41.
Local pharmacist Mahendra Patel was third on 479, followed by Colin Leather (108), Carol Quine (43) and David Pownall (33).
The turnout in Castletown was just over 65 per cent.
Mr Ronan is the constituency's first new MHK for 30 years, following the recent retirement from politics of Tony Brown, the outgoing chief minister.