Either way, a British victor is assured, a positive for home fans after the defeat of Andy Murray in the quarters and Johanna Konta in the singles semis.
And Murray saved break back points in the final game on his serve before sealing the match after 71 minutes.
JAMIE MURRAY put a British seal on Wimbledon by winning the mixed doubles title with Martina Hingis.
Another Murray intervention sealed a second break for 4-3 and this time they served out when the Scot hammered away a smash.
The second set was close until the seventh game when Watson's serve came under pressure. But it was a great opportunity.
The win is Murray's fourth in total at a grand slam, meaning he leads his brother Andy by one heading into the US Open, and the younger Murray was at SW19 to see his sibling triumph.
Hingis revealed she had a nervous wait to find out whether Murray would agree to the partnership.
With Murray and Watson on opposite sides of the net, it was a win-win situation for British fans on Centre Court, most of whom had stuck around after seeing Roger Federer clinch a record eighth singles title earlier in the day. "She's an wonderful player", he said before the final. I was like, Oh, my God, did he read it (her text). "He left me hanging overnight", Hingis said, laughing. I understand in the past guys would have said no because they really want to focus on [men's] doubles, only because it's three-out-of-five [sets].
"I mean, the (men's) doubles for me is obviously my biggest goal of the year", Murray said.
Depending on the stage of a player's career, though, mixed doubles can be more than just the chance to top up the bank balance. "I don't know if he snuck a few points in or not".
Watson and Kontinen suffered defeat for a first time together and the Brit admitted it was a "strange feeling".
Murray said: "It's just a great opportunity for me to play and to, like, really feel like you could do good things on the court and have a great shot to win".
Watson said: "It's been a great tournament all around".