Townend described Mullins riding as a "privilege"
The 12-1 chances of Al Boum Photo, driven by Paul Townend, gave coach Willie Mullins a first win at the Cheltenham Gold Cup.
Mullins, the most successful coach of the Festival, had already finished second on six occasions in the prestigious race.
Al Boum Photo had moments of anxiety with his jump. but ran far to win by two and a half lengths.
His Irish compatriot Anibale Fly (22-1) finished second with Bristol De Mai Gray (18-1), third.
"I can not believe it," an excited Citizen of Townend told BBC Radio 5 Live. "It means everything – it's the pinnacle and I can not believe I won it.
"I'm in a good rhythm, and when you're doing well, it seems so simple."
But Mullins had mixed feelings after the invitation of one of his three other riders, victim of a fatal injury, following a fall in the race.
This was the second horse lost by Mullins to the Festival after Ballyward on the first day and after the fall of Sir Erec, coach of Joseph O. Brien, in the first race of the day.
For a long time, it seemed that last year 's winner, Native River, and runner-up Might Bite would be in turmoil again while they were leading the standings after the first circuit.
The Clan Des Obeaux, which belongs in part to Sir Alex Ferguson, former manager of Manchester United, seemed to take up the challenge.
But it was Al Boum Photo, the only one of the four Mullins riders to reach the second circuit, which started to appear the strongest.
Both men had a slight flicker at the penultimate before negotiating safely the last fence and fighting for victory on the famous Cheltenham Hill.
"I'm used to the disappointment of finishing second and I kind of resigned myself to never winning the Gold Cup," said Mullins, whose father, Paddy, had coached Dawn Run to victory in 1986.
"To have three of my four riders out of the race before going to the penultimate first lap, I was counting on one.
"Every time I looked at Paul, his body language told me that he was very happy and that the horse was relaxed.
"When he came to the end of the game, I just looked at the winning post and thought nothing would come looking for it unless it stopped or ran or something like that.
Townend and Al Boum Photo had been at the center of a dramatic incident at the Punchestown festival last year that had left the jockey with a 21-day ban for dangerous driving.
Townend, while leading the hunt for the Novice Champion, led the horse to the right of the last hurdle after apparently hearing a scream telling him to get around it.
During the process, he collided with Oscar, his fellow runner, and crashed against the white fence.