It was also the second time Paxton, the world number two, had taken a title against top seed Paul Foster, having won the men's pairs on Monday.
And it denied Scotsman Foster and Welsh partner Laura Thomas a record third straight title.
Englishman Paxton and his Welsh partner won a thrilling final 8-10 9-7 2-1.
Just like the men's pairs final, Paxton fought back from a set down - and 1-0 down in the tie break - to claim victory.
But he admitted it needed a crucial fluke shot in the final end of the second set to prevent a defeat.
"It's a shame for the game when that happens because it was a great game of bowls," Paxton told BBC Sport
"To get into a tie break like that, no player likes to and I still feel bad about it, but it was a great match."
The Englishman, who took the men's pairs title along with Simon Skelton as they defeated Foster and Alex Marshall, played down the significance of two victories in a row over the Scot.
"You never get the better of Paul," said Paxton. "He's always there to knock you back - it's one game, that's all."
Purcell was overwhelmed by the victory, adding: "I cannot believe it really - it's wonderful."
Foster was disappointed to lose in such a manner.
"That's three years in a row we've been in the final and Laura's been a fantastic partner," he said.
"It is part of the game and you've got accept it, but it's hard to take.
"Yesterday hurt and so does today, but you have to move on.
"Congratulations to Robert and Marion. It was a great final."
Foster and Paxton could face each other in the men's singles final should they both progress in Norfolk.
Robert Paxton claimed his second World Indoor Bowls title in two days as he and Marion Purcell won the mixed pairs at Potters in Great Yarmouth.