The John Gosden-trained colt just got the better of Aidan O'Brien's Nelson as he supplemented wins at this track and Kempton to take the Group Two spoils.
Nelson and Mildenberger jointly made the running, with Midnight Wilde keeping them company until he was outclassed as they approached the bushes.
At that point, Roaring Lion (11-4) burst on the scene under Oisin Murphy and he dug deep to outpoint Nelson by a neck, with Mildenberger third.
Roaring Lion was given a quote of 25-1 for the Derby with Paddy Power.
Murphy said: "I was still on the bridle passing the bushes, so I couldn't have sat any longer.
"We went a good gallop and he travelled like a really nice horse. I must be the luckiest young lad in the world riding horses like this.
"I haven't ridden a Derby horse in a while, so I'm not sure if he is one or not, but he certainly has a big future."
Winning owner Sheikh Fahad said: "John has done a great job with the horse and Oisin has ridden him brilliantly there.
"He travels well, so it's exciting. You've got to dream. It's great to go into the winter with a horse like him."
Gosden said: "He did very well and it wouldn't be his favourite ground. He has done nothing but improve. He has won on the July Course and the Polytrack and we couldn't be more pleased with him. He came smoothly through (with) hands and heels to win it.
"It's a big track and he is a baby. He was in the middle and he had no rail and he started wandering.
"Oisin gave him a flick with his right hand and he was nearly in the grandstand. He straightened him up and he came back as he went a neck down. He has got plenty of class and is a nice horse.
"You either put him away or you look at a choice of two races. One is obviously Doncaster and the other is Del Mar.
"I love Doncaster and I would be worried if it got deep and that wouldn't be the place to go. He is an American-bred horse and he likes top of the ground. The next 10 days with the horse will tell us everything.
"He has come on a lot from Kempton. I would have been terribly disappointed if he wasn't in the shake-up."
Paddy Power cut Roaring Lion to 6-1 from 10-1 for the Breeders' Cup Juvenile Turf at Del Mar.
O'Brien said of Nelson: "Ryan (Moore) rode him as he found him. He ran a very good race and was right there all the way.
"He had a bit of a messy morning and he only just arrived at the races. The poor fellow had never flown before, but he learnt a lot today. I don't know how much that affected him, but it wasn't an advantage.
"He is a little bit of baby and took a few runs to learn. We are very happy with the run. He kept going and he is a nice horse.
"Ryan was happy with him and that he ran a good, solid race. If he was to run again, he would have the choice of the Racing Post Trophy or the mile-and-a-quarter race in France."