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 France, was fourth at 50/1.
Delacroix, who had beaten 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.
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 knew I'd gone a good gallop but his ears were pricked and I knew he had plenty left. He's a horse we always thought stays well so I thought for anybody to get to me they would have to stay well and it would be tough for them.
