The reigning Olympic champions beat Timea Babos of Hungary and Kazakhstan's Yaroslava Shvedova 6-3 6-4 on Centre Court -- hours after Serena claimed a record-equalling 22nd grand slam singles title by defeating Germany's Angelique Kerber 7-5 6-3.
Serena has now won both the Wimbledon singles and doubles (with Venus) in the same year on four occasions. Together, the American duo have a perfect 14-0 record in grand slam doubles finals.
"It's unbelievable," said Serena. "It's a feat even I couldn't have thought would have happened. It's amazing."
For Venus it was a consolation after she lost to Kerber in Thursday's singles semi-finals - denying her a first grand slam singles final since losing to Serena at Wimbledon in 2009.
"It's a privilege, it's an honour," said Venus. "When you enter the doubles draw, you dream of winning. Then we enter the singles, we dream of winning.
"To have Williams on both of those, somehow in 2016, it's another dream come true."
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The sisters, who paired up again this year after a two-year absence, have won 14 grand slam doubles titles together -- moving them level with American Gigi Fernandez and Belarussian Natasha Zvereva in second place on the all-time list.
They still have some way to go, however, to match the 20 of Martina Navratilova and fellow American Pam Shriver.
"I didn't know about that stat until yesterday, that we were 13-0," said Venus. "It's not something you really think about. You just get to the final and try to win.
"I don't think it made us nervous. You got time. If you get down a break, you always have time to get the break back. You can always go to a third (set).
"When you're out there, you probably realise you have a chance to go for it again."
The Williams sisters are now 22-1 in all doubles finals they have played together.
