Lambourn produced a powerful front-running display to provide trainer Aidan O'Brien with an eleventh win in the Derby.
Lambourn, winner of the Chester Vase on his previous outing, was sent straight into the lead by Wayne Lordan and had matters in control a long way out.
The 13/2 shot won by three and three-quarter lengths from 50/1 chance Lazy Griff who had also chased Lambourn home in the Chester Vase.
Tennessee Stud was third at 28/1, while New Ground, one of two contenders from , was fourth at 50/1.
Delacroix, who had beaten his stablemate Lambourn in the Ballysax Stakes at Leopardstown in March and was sent off the 2/1 favourite under Ryan Moore, never threatened to make an impact from the rear and finished ninth.
The absence of 2000 Guineas winner Ruling Court meant that 18 set off in the Derby but, despite the large field, very few got involved.
Lordan, riding a horse with proven stamina, kicked on early in the straight and moved clear entering the final three furlongs. Everything in behind was hard at work by that stage and Lambourn kept up the relentless gallop to win unchallenged.
It was a first Epsom classic victory for Lordan who had been narrowly denied in the Oaks on Friday when Whirl was beaten a neck by stablemate Minnie Hauk.
The winning rider said: "I never thought it was going to be as easy as it was, but I was happy to take it. He was out in front, going a good even gallop, his ears were pricked and he was obviously enjoying it. I’ve seen from his other races that he saves plenty, so I was happy to go forward really early, and I thought he got to the line really well. He’s a horse who is going to stay well, so I think going forward is the option with him.
"When you are growing up the Epsom Derby is always the one. When you get a ride in it you are happy, when you get on one that you think has a chance you are pleased, but when they come and win it it’s special.
"I suppose the one for me was always the Breeders’ Cup on Iridessa, but I think this is going to top it. I’m in a privileged position with Aidan at Ballydoyle to get the chance to ride horses like this."
O'Brien hails Lambourn's attitude
Lambourn's win capped a superb meeting for O'Brien who landed Friday's two Group 1s with Jan Brueghel (Coronation Cup) and Minnie Hauk (Oaks). O'Brien first won the Derby in 2001 with Galileo, the grandsire of Lambourn, and also won it in 2014 with Lambourn's sire Australia.
The winning trainer said: "He's very straightforward and Wayne knew that he would stay. He's a very fit, genuine horse, a typical Australia.
"Australia was a great Derby winner, very genuine, and that's what this horse is: uncomplicated, genuine and committed. He always has been.
"His runs have been very good. He was second to Delacroix and then he went and won very impressively at Chester. At Chester you have to be on your game and stay, and that's the way he was. Ryan loved him at Chester but obviously couldn't ride them all."
On the winning rider, O'Brien said: "Wayne's been a great fella, he's done so much hard work through the years. He gave him an incredible ride, everybody knew what he was going to do."
Lambourn has been cut to a general 5/2 for the St Leger and is 14/1 for the Prix de l'Arc de Triomphe. He's been introduced as the 4/5 favourite for the Irish Derby with Paddy Power.
Placed horses please connections
Lazy Griff, ridden by Christophe Soumillon in the silks of Middleham Park Racing, may have been sent off at a big price but his brave effort in second didn't come as a surprise to his trainer, Charlie Johnston, who said: "Fantastic run. I couldn’t say it enough times, that I thought the horse was overpriced and unconsidered. That Chester Vase form looks alright now, doesn’t it?
"And, to be honest, I didn’t think he’d love the track - I thought he’d be better in Ireland or better in Doncaster, so there’s lots to look forward to. The Irish Derby would be an option."
Joseph O'Brien was also pleased with how his runner, Tennessee Stud, performed in third, and said: "He ran great. He was a little bit slower into his stride than we maybe expected, but it was a really good run. Very proud of the horse, a great ride from Dylan and I’m delighted for all of the owners."
Aidan O'Brien said that Delacroix met with early interference and that Moore reported his chance was over from about halfway, while his other runner, The Lion In Winter, was also struggling from a long way out before ultimately finishing fourteenth.
Dante winner Pride Of Arras beat only one home - the Dante runner-up Damysus - and his trainer, Ralph Beckett, reported he failed to handle the track. Beckett's other runner, Stanhope Gardens, gave a much better and only just missed out on fourth.
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