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 Jamin De Vaux in the Albert Bartlett Novices’ Hurdle.
