Alan King's Group One victor was carrying 10st 8lb, meaning he gave up upwards of 19lb to his Handicap rivals.
Nevertheless, the money flooded in for Trueshan as he was sent off as favourite under Hollie Doyle.
As the runners fanned across the track, Doyle was niggling away with over half a mile out just to hold position. The horse looked booked for a place at best as Spirit Mixer pulled in-front.
However, Trueshan got about his business in fine style, and closed the gap before sensationally winning the race by half a length.
This was Doyle's first success since the French Oaks last weekend, with the jockey saying: "It's been a good few months, or years I could say.
"I can't complain how things are going but I'm pretty greedy, once I get a taste of success I want more and more. I'll do all I can to keep it going. Long may it continue."
Of Trueshan, she added: "That was unreal. But I had to give him a squeeze and I don't usually have to do that. He's never had to battle hard to win a race, he's always won so easily.
"But he got into that battle and it was the first time I've ever seen it from him, but his ears pinned back and he really knuckled down. He was not getting beaten."
Trueshan finished sixth under a big weight in last year's Northumberland Plate before going on to win the Goodwood Cup, the Prix du Cadran and the Long Distance Cup on Champions Day at Ascot.
Part-owner Alan Marsh will be doing a rain dance to allow the six-year-old to defend his Goodwood crown on July 26.
He said: "It was an extraordinary win. Hollie told us exactly how she was going to ride him and she did it. We just tell her to get on with it and she does it. She knows the horse very well. All credit to her.
"I didn't think he would win, of course not. It's well documented we were running because we couldn't run at Ascot, we are aiming for the Goodwood Cup and we needed a run. This was the safest place to run.
"I only decided to come at 3pm yesterday and I got to Kings Cross and found there was a train. I'm the only one of the four owners here. It's a delight. Thank goodness they can't put us up for the Goodwood Cup!"