McEwan was the epitome of consistency for the first three rounds with 72, 72 and 71, but he saved his best for last.
He holds the course record of 63 and came close to this in the final round, carding 64 to finish 13 shots ahead of second-placed Aaron Bailey (Gleniti).
Young Balmacewen golfer Brandon Hodgson had his best performance in Otago trials, shooting a 293 total for third place.
McEwan held a two-shot lead going into the final round but made certain of the title by birdieing five of the first six holes.
The only blemish came with a bogey four at the seventh, but he soon made amends with a birdie four at the eighth to put him 5 under at the turn.
In a faultless homeward nine, McEwan birdied 10, 12 and 16 for his 8-under 64.
Ken Shaw (Taieri) and Chris Bond (Waitikiri) opened the scoring with rounds of 3-under 69, while Andrew Hobbs (St Clair) finished one shot back.
But in the afternoon, all three faltered, with Shaw and Bond carding 79 and Hobbs making 78, with all three five shots off the pace.
This left Hodgson with the overnight lead followed by McEwan, who was one shot back.
Luke Murdoch and Waitaki schoolboy Andrew Caldwell remained in contention three and four shots off the pace respectively.
But with 71, his first assault on par, McEwan took a two-stroke lead over Hodgson, with former Oamaru golfer Bond in third place.
