Michael Smith (Otago) also scored a one under par 70 in the morning round but finished two shots behind with a par 71 in the afternoon.
Both players have a liking for the links course and have both featured in tournaments at the Park in the past.
Mark Collie (Chisholm Park) finished in third place two further shots back while a consistent and underrated Stephen Hitchcox (St Clair) finished two over par to finish fourth.
North Otago greenkeeper Gary Creedy was the best of the masters (over-40) golfers coming in fifth with rounds of 71 and 74.
The open tournament attracted three professional entries with Otago's Mahal Pearce taking the honours on 139 - the same score as the winning amateur.
Craig Palmer (The Hills) finished in a tie for fourth while Alan Rose (Sharpies) was two further shots back.
With a three under par 68, Pearce posted the low score of the day while McEwan's afternoon round of 69 was the key to his win.
In the morning, McEwan made four birdies on the front nine turning three under par, but three bogeys coming home proved costly.
Smith dropped a shot on the second hole but birdies at the seventh and ninth holes put him under par.
His homeward nine was the epitome of consistency with nine straight pars.
McEwan and Smith both went out two under on 33. McEwan had two birdies and seven pars while Smith made an eagle, two birdies and two bogeys.
Par three holes were the saviour for McEwan on the final nine as his bogeys at 11 and 15 were cancelled out by birdie two's on the two par three holes.
Smith had every opportunity to take the title leading by one with four holes to play.
But he bogeyed three of the last four holes which cost him dearly.
The next Otago men's trial fixture will be played in Wanaka next weekend.
