Poniros stunned a capacity crowd as the longest-priced winner of the Triumph Hurdle.
There were gasps as his odds were announced following a late charge under jockey Jonjo O’Neill Jr.
Mullins is the dominant trainer at Cheltenham, but his winner was running over hurdles for the first time and considered an outsider among his 11 contenders in the 18-runner opener.
Poniros, running in the blue-and-white colours of Brighton and Hove Albion owner Tony Bloom, won by a neck from Lulamba with favourite East India Dock in third.
“I know my owner probably had some pounds on it but I didn’t give him any advice. I don’t think I’ve ever given Poniros a serious gallop, it was more for a nice experience for him,” said Mullins.
“He’s done a good bit of jumping at home. We gave him a break and brought him back in for the spring. I didn’t think he’d be sharp enough for this.
“I saw the blue coming and was thinking’ is that one of mine?’ That shows where his Flat racing experience comes in for this.”
O’Neill, who was due to be riding at Doncaster instead but got a late call-up, said: “It’s crazy. It’s a funny old game. Anything of Willie’s has a chance.”
Mullins followed up with shorter-priced winners in the County Hurdle and Mares’ Chase as Paul Townend and Mark Walsh respectively steered favourites Kargese (6-4) and Dinoblue (6-4) to victory.
Townend claimed the fourth Mullins winner aboard 6-1 shot Jasmin De Vaux in the Albert Bartlett Novices’ Hurdle.
Wonderwall held off the challenge of Its On The Line to win the Hunters’ Chase for trainer Sam Curling and jockey Rob James at 28-1.
And in the final race of the meeting, Gordon Elliott finally got a winner after four second places over the four days as Wodhooh (9-2) took the Martin Pipe Hurdle for jockey Danny Gilligan.
That was a clean sweep on Friday and 20th victory of the week for Irish-trained runners – 10 of them courtesy of leading trainer Mullins, whose individual tally was two more than all of the British contingent.
Cheltenham reported a crowd of 68,026, taking the total attendance to 218,839 for the week, compared to nearly 230,000 last year.
