The 10-year-old was among the market principals at 9-2 and was given a patient ride by Mullins junior, who was bidding for his second victory in the race following the success of Back In Focus five years ago.
The front-running Sizing Tennessee and Ms Parfois led the field into the straight, but Rathvinden was travelling ominously well in their slipstream and moved to the front after the second-last.
Ms Parfois refused to go down without a fight under Will Biddick, but the Mullins runner won the argument by half a length.
Sizing Tennessee was a long way back in third.
Rathvinden had to be dismounted after the line and was doused with water by the veterinary team.
He did not return to the winner's enclosure, but walked back to the racecourse stables seemingly none the worse.
Mullins senior said: "I've just spoken to the vet and he said he's fine, just exhausted. He felt it was safer to take him back to the stables than the razzmatazz of the paddock.
"They'll just throw lots of water on him. He's taking the odd step, then a wobble. I imagine any athlete after four miles would be a little tired."
On the race itself, he said: "I was happy enough watching because I can tell by Patrick's body language how he's going. I thought he might win it easier but the second horse put up some fight.
"They pulled a long way clear and it was a good finish after four miles on that ground."
Mullins also struck with Footpad and Benie Des Dieux, and added: "I'm delighted to have three winners, I'm just happier the horses are starting to perform.
"He could be a Grand National horse next year. He jumps and stays."
Mossback had to be put down after suffering a shoulder injury.
Patrick Mullins received a six-day suspension for using his whip above the permitted level and without giving his horse time to respond from the last fence.