With the rain-softened ground leading to the withdrawal of four-time Goodwood Cup hero Stradivarius, Alan King's charge was the 6/5 favourite to provide his multiple Grade One-winning trainer with a first top-level success on the Flat.
Trueshan raced keenly for much of the two-mile journey, but moved to the lead early in the home straight and saw off the brave effort of the largely-unconsidered 33/1 shot Away He Goes by just under four lengths.
Trueshan was a runaway winner on Champions Day at Ascot in October, the five-year-old made a promising start to the current campaign when runner-up to Japan at Chester in May before missing an intended appearance in the Gold Cup at Royal Ascot due to unsuitable ground.
Trueshan instead contest the following week's Northumberland Plate, where he was far from disgraced in finishing sixth under a huge weight, but everything fell perfectly into place for him on the Sussex Downs.
Doyle said: "It's incredible. He's been in my mind every day since Champions Day, when we could get back together - these are the days you do it for.
"He was pretty fresh early on, they were going no gallop and I expected there to be a bit of pace on and at halfway I thought 'I've got to do something about this, I'm not getting trapped on the rail, they're going to sprint finish'.
"I managed to slide onto the girths of the leaders, he completely dropped the bridle with me and the further he went, the better. When I hit the rising ground, he went again.
"It's a staying challenge and when I hit the rising ground he was gone again.
"He's a superstar on this ground. I got some buzz off that - I don't get too high or too low, but when you get experiences like that you've got to make the most of it."
King is perhaps more renowned for winners at Cheltenham, but is becoming an increasingly potent force on the Flat.
He said: "It's no better but it's right up there with the Champion Hurdles and things, of course it is.
"All I want to do is train proper horses and I don't mind whether they're jumpers or not, I'm not giving up the jumping just yet!"
He went on: "We're out of practice a bit, I haven't been coming racing much over the past 18 months and I don't think I've ever saddled this horse to win - I was nearly told to stay at home!
"He was just a little bit keen but he can be like that - thank God we went to Newcastle because if he'd come straight from Chester he'd have been ferocious today.
"I was glad he had a proper race there and we were able to let him down and then build him back up, a lot of people thought we were mad running him (at Newcastle) without a flat weight but he needed to go somewhere."
Trainer Ismail Mohammed was thrilled with the effort of Away He Goes.
He said: "As we watched we thought he was going to win. We are very proud of him. Maybe we will head to France for a race one month from now that we had pencilled in."
His rider Jim Crowley was equally pleased, saying: "He was travelling better than the winner who outstayed him on that ground. His form in Dubai was very good, so it was not a total surprise."