The 67-year-old emulated compatriot Vincent O'Brien -- regarded by many as the greatest trainer of all time -- who won the second of his two titles in 1954 before going on to dominate on the flat.
Mullins rubber stamped his supremacy in the big races in the United Kingdom as his Minella Cocooner just got up to win the feature handicap Chase at Sandown racecourse in England, with Impaire Et Passe landing the big hurdle.
"Awesome," Mullins told ITV at Sandown on being told he was assured of being champion prior even to Minella Cocooner winning.
"Vincent (O'Brien) is the legend of the game and just to emulate something he did is extraordinary and something we couldn’t comprehend doing.
“We had never dreamed of doing it but after we were pipped at the post in 2016, I said if we ever got the opportunity again then we would go for it. David Casey (assistant) went through all our horses to find out what was qualified for all races in England, Scotland and Wales!
“What more can I say? I am amazed at the racing public over here, to be welcomed with such open arms everywhere we have gone.
"But here we are, owners have been supportive, as have the staff and jockeys and we have had a team of horses to race over here.
"A great team all round."
Mullins, who sent a team over for Sandown's climax to the season Finals Day, said it had never been something he had thought about.
"It is something different to win.
"Being Irish champion trainer yes, but never dreamt of winning the British one.
"To do it against Nicky Henderson, Dan Skelton and Paul Nicholls, all very good trainers."
"It is probably tough on Dan, he beat Paul Nicholls and we come up the inside.
"We are very lucky to have a tremendous team of horses at the moment."
Mullins is way behind his two rivals Skelton -- son of 2016 Olympic showjumping champion Nick -- and Nicholls in terms of winners but trumps them on the deciding factor, prizemoney.
Mullins's 28 winners in Britain netted almost 3.5 million euros in prize money.
This haul has been based on wins in the Champion Hurdle, Cheltenham Gold Cup -- he also brought up a century of Festival winners -- and the Grand National with I Am Maximus.
That for once in Mullins's case is not a milestone treble as his fellow Irishman Henry de Bromhead won all three plus the Queen Mother Champion Chase at the Cheltenham Festival in 2021.
He also won last Saturday's Scottish National with Macdermott and the same jockey Danny Mullins was on board Minella Cocooner.
He was in awe of his uncle's feat.
"It is something that should not be achievable," said Danny.
The rare achievement is due to marked in some style, Mullins said.
"I am sure we are going to celebrate long and hard! We will start off this evening, go on through the whole of Punchestown next week and maybe a few days after that!
"Everyone at home at Closutton (his stable in County Carlow), the local community and the whole of Irish racing has really been hyped by the whole thing and wishing us the best of luck for today."
