The unseeded Watson and Kontinen, playing in their first event together, beat 15th seeds Anna-Lena Groenefeld of Germany and Colombian Roberto Farah 7-6(5) 6-4 on Centre Court after Andy Murray had clinched his second men's singles crown.
"I guess this is the tournament that every player has in their mind, the prestige and everything... It's a special place to win," said Kontinen, whose previous best in mixed doubles was reaching the 2015 French Open semis with Zheng Jie of China.
"I'll remember this forever," added British number two Watson. "It's been a dream of mine since I was a little girl to be a grand slam champion. I would take anything, singles, doubles, mixed doubles... I've got one of those now."
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(Reporting by Rex Gowar; editing by Ken Ferris)
