Rufus McLean, another wing, crossed for two tries in the opening 15 minutes, while prop Pierre Schoeman also marked his first cap with a first-half try.
There were second-half tries from George Turner and substitutes Nick Haining and Oli Kebble as Scotland romped to victory in its first match in front of a crowd at Murrayfield in 19 months.
Steyn was starting for the first time for the Dark Blues and became the first Scotland player to score four tries in a match since Gavin Hastings against Ivory Coast at the 1995 Rugby World Cup.
The Scots are set to face tougher matches against Australia, South Africa and Japan over the next three weekends, but will have their England and France-based players available.
Tonga's only try came through David Lolohea – and that was scored while Scotland was down to 14 men following Rob Harley’s sin-binning.