The London 2012 bronze medallist led throughout to win in 7:08.31 minutes.
Campbell, who has become the first Northern Ireland athlete to compete in four Olympic Games, goes through to the quarter-finals on Tuesday.
Fellow Coleraine man Peter Chambers races in the lightweight fours first round later on Saturday.
Clare Abbott begins with the three-day event dressage and several Ulstermen will play for the Ireland and GB hockey teams.
The world-ranked 12th Ireland men open their campaign with a vital game against fifth-ranked India at the Olympic Hockey Centre at 15:00 BST.
The Irish squad includes Lisnagarvey trio Jonathan Bell, Michael Watt and Paul Gleghorne, whose brother Mark is part of the British squad.
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Eugene Magee, Peter Carruth, Chris Cargo and John Jackson are the other Ulstermen in the Ireland squad while Timmy Cockram and Michael Robson have travelled to Rio as reserves.
With tough games against the Netherlands (Sunday) and defending champions Germany (Tuesday) coming up over the next four days, the Irish will be looking for a positive result in Saturday's opener.
Ulster quartet Iain Lewers, David Ames, Ian Sloan and Mark Gleghorne will be part of the GB squad which faces Belgium at 16:30.
The Belgians are currently ranked sixth in the world two places behind England, who supply 12 of the 16-man squad.
Hillsborough-based Abbott, who celebrated her 30th birthday last week, will be beginning her first Olympics as she completes for Ireland in the three-day event team event.
The dressage component is scheduled to get under way in Rio on Saturday at 14:00 BST.
Coleraine rower Alan Campbell made a winning start in his Olympic Games bid with a comfortable single sculls heat victory in Rio on Saturday.