Harris led a seven-play, 67-yard drive in the fourth quarter, capped by a two-yard TD run by C.J. Gable to seal the win and move Edmonton to 5-3 and into a logjam for second place in the West Division.
It was otherwise a frustrating night for Edmonton’s offence.
The Eskimos turned the ball over three times on third down and had to stop the Redblacks on three plays in the shadow of their goalposts as time ticked down to preserve the win.
"It’s not pretty, but two points is two points," said Harris.
"If we can stop shooting ourselves in the foot, we can be pretty good."
Harris came to Edmonton as a free agent this past off season after three years in Ottawa.
Asked about playing his old team, he said "I’m an Eskimo.
"That was a past life.
"They’re doing their thing and we’re doing ours."
Ottawa is 3-5 and third in the East Division — and things may get worse.
The Redblacks’ star kick returner DeVonte Dedmon had to be helped off the field late in the fourth quarter after getting injured in punt coverage and couldn’t put weight on his left leg.
The Eskimos were paced by Gable, who ran for 116 yards on 18 carries and two touchdowns.
Harris finished with 33 of 40 in passing for 327 yards.
Ottawa quarterback Dominique Davis was 23 for 35 for 289 yards, to go with one touchdown and two interceptions.
It was a chippy affair under cloudy skies before 27,951 fans at Commonwealth Stadium, featuring multiple pushes and shoves and the odd haymaker punch after the whistle.
The teams are well acquainted. There are 13 former Redblacks on the Eskimo roster.
The Eskimos opened the scoring with about six minutes to play in the first quarter.
Edmonton capitalized on an Arjen Colquhoun interception to drive 68 yards on eight plays to paydirt, capped by a 28-yard touchdown run up the middle by Gable.
The Redblacks responded with a Lewis Ward 10-yard field goal followed by a deep Davis pass to a streaking Dominique Rhymes down the middle for a 67-yard TD. But Ottawa then failed on the two-point convert.
The teams swapped field goals in the second quarter.
Edmonton’s Sean Whyte hit from 44 yards but also saw a 45-yard attempt clang off the upright.
Ward kicked a second three-pointer from 16 yards out to extend his CFL-record consecutive field goal streak to 69.
After a scoreless third quarter, Edmonton retook the momentum for good in the fourth when Harris drove the offence for the major at the 6:24 mark.
The two-point convert failed, leaving the Esks up 16-12.
Edmonton head coach Jason Maas said Harris managed the game well.
"We were taking what the defence gave us. Trevor did a good job with that all night," said Maas.
"He made good decisions and kept the ball out of their hands."
Edmonton’s defence held off a late-game charge by the Redblacks, who managed to drive the ball to the Eskimo 11 yard line before stalling with 31 seconds left.
There was no immediate word on Dedmon, who electrified the league last week when he ran back two kicks for touchdowns against Montreal.
The Eskimos managed to keep Dedmon in check. His best run was 41 yards on a kickoff in the first half, but he was otherwise bottled up.
Kick returner Christion Jones showed promise in his Eskimo debut, including returns of 27 and 28 yards in the first half.
Jones was acquired this week from Saskatchewan for receiver Kenny Stafford after kick returner Martese Jackson was placed on the six-game injured list.
Edmonton now heads to Toronto to play the Argonauts Friday.
Ottawa hosts the Hamilton Tiger-Cats on Aug. 17.