Lucarno overcame the challenge of Aidan O'Brien duo Mahler and Honolulu to win the St Leger at Doncaster, the world's oldest Classic.
Jimmy Fortune powers to the line for his first Classic victory
The 7-2 second favourite overhauled Mahler late on to give trainer John Gosden and jockey Jimmy Fortune a famous win.
The three-year-old, who triumphed by a length, travelled well and once he got to the front, he looked strong.
Mahler had to be content with second, just ahead of the favourite Honolulu.
Mick Kinane set a tough early pace on Mahler, who was always likely to stay but as soon as Lucarno, who was fourth in the Epsom Derby, got through he had the pace to win.
Fortune, who was celebrating his first Classic win, said: "He's made up into a very good horse. We had a little doubt about him staying but he stayed well.
"They went a good pace and I was happy to sit off it and we picked them off one by one in the straight. "He's done it very nicely. He's a beautiful horse, very relaxed and a real professional.
Winning trainer Gosden said: "He has run well all year.
"He made his debut in what was probably the hottest maiden of the year at Newbury and he finished fourth in what was probably a very good Derby.
"He has always been a very good horse. We will put him away now and he will be a lovely horse for next year."
Meanwhile, Aidan O'Brien got a long-awaited first success in the Irish St Leger when the 4-7 favourite Yeats beat stablemate Scorpion by half a length.
Yeats, ridden by Kieren Fallon, sat off the pace set by Scorpion but when Scorpion went clear with two furlongs to run, Yeats was slow to react.
Fallon had to get deadly serious with his mount and his persistence paid off as the dual Ascot Gold Cup winner triumphed ahead of Scorpion with the three-year-old Mores Wells in third.
St Leger (1m 6½f)
1. Lucarno (J Fortune) 7-2
2. Mahler (M J Kinane) 13-2
3. Honolulu (J Murtagh) 13-8 Fav