The 28-year-old was working hard to get his teammate and fellow Norwegian Edvald Boasson Hagen onto the top step of the podium, but ended up winning a frantic final sprint to claim the biggest victory to date in his three-year pro career.
Nordhaug, of the powerful Sky Procycling team, came in two seconds ahead of rising Italian prospect Moreno Moser and Russian Alexandr Kolobnev in the top-level UCI World Tour event.
"I was helping Edvard," he said. "I attacked up the climb with Edvard on my wheel and we were in a big group.
"Then I just tried to cover moves so no-one escaped, but with three kilometres to go, I got a little gap and Edvard was screaming on the radio to go. That's when I went. I thought I was going to get beaten in the sprint, but I won."
Australian Simon Gerrans, the Quebec City winner, came in fourth with Boasson Hagen fifth. Defending champion Rui Costa of Portugal was eighth.
Ryder Hesjedal, who ran out of steam on the final lap of a race in Quebec City on Friday, was in the lead group until the final kilometre, but fell back to 23rd spot. The Garmin-Sharp team leader from Victoria was top Canadian just ahead of David Veilleux of Cap Rouge, Que., in 24th and Francois Parisien of Montreal in 25th.
Team Sky has dominated the sport this year, with star Bradley Wiggins taking the Tour de France, but Nordhaug has already agreed to a two-year deal with the Radobank squad for next season.
Reports said that because of his decision he was left off the team for the Vuelta, or Tour of Spain, which ended on Sunday with Alberto Contador the winner.
But at least he will leave behind a victory in a tough one-day race.
"We are a good group and I'm super happy at Team Sky," said Nordhaug. "I made my choice to leave next year, but now I'm only thinking about the rest of the season. We're good friends here and I'm happy I won."
He may not be the only one to leave. Reports indicate reigning world champion Mark Cavendish has asked to be let out of his contract to find a new team with more opportunity to be lead rider.
The 21-year-old Moser, whose father Diego was a pro rider and whose uncle Francesco won the Giro d'Italia in 1984, has wins in the Tour of Poland and a one-day event in Frankfurt in his rookie season on the UCI World Tour.
Much like Nordhaug, he was at the front of the race trying to get Liquigas-Cannondale team leader Peter Sagen into position for the win, and ended up on the podium.
"At the beginning of the season, I didn't think I'd have a season like this so I'm very happy," he said.
It was the final race on Canadian soil for 36-year-old Michael Barry of Toronto, who plans to retire at the end of the season after a long career of top-level racing. He will officially call it quits after the Tour of Beijing next month. He finished in the main peloton in 57th place.
"It was tough, emotional and special," said Barry, who raced for 14 years with top teams in Europe, mostly as a domestique, or helper. "At the start of the season, these races were an objective of mine."
It was ideal 19 C racing weather, although quite windy at the top of the course for the 205.7-kilometre race, which saw the 21 teams do 17 laps of a steep course that ran up and down Mount Royal in the centre of the city.
All the action came on the last lap, as several breakaway attempts were made. Hesjedal helped chase down the first two, but Canada's top rider couldn't keep up the pace.
Hesjedal had bad luck after he became the first Canadian to win a major Tour with his Giro victory in May. He was in top form and in eighth place overall at the Tour de France in July when he suffered leg and hip injuries in a crash and had to withdraw.
Since finishing 62nd at the London Olympics more than a month ago he had not raced before the Quebec City event.
"If I look at how I was on Friday, I wasn't certain how I would be able to perform," Hesjedal said. "I went for it and I was happy with what I was able to do.
"I committed to the effort and was able to stay ahead of the curve a bit and keep myself in the final and give us a chance."
Hesjedal finished third at the inaugural Montreal race in 2010.
"I'd like to feel that again and do even better and win this race, but you have to be realistic and see where you are in your season," said Hesjedal, who hopes to improve his conditioning for the world championships in two weeks in the Netherlands.
"I won the Giro and held that form for the Tour de France before i crashed and had to leave. Then I had to recover and move back to training."
The entertainment for the crowd lining the course came from Eduard Vorganov, Manuele Boaro and Egoi Martinez, who broke away at the start of the second lap and built a more than seven minute lead.
Boaro was dropped along the way, but Vorganov and Martinez put up a valiant fight before they were swallowed by the pack on the next to last lap, with the Canadian SpiderTech team doing much of the work at the front of the peloton.
The Quebec City and Montreal events are the only World Tour races held in North America. The field included six of the world's top 20 riders.