The pair easily beat the defending champions, Taiwan's Hsieh Su-wei and Peng Shuai of China, 6-1 6-0, reeling off 12 games in a row after dropping the opening game.
For Black, it was her third win in the elite event, and her first since she returned to the sport after taking time out to have a child.The 35-year-old doubles specialist previously won with Liezel Huber in 2007 and 2008.
She and Mirza, who shared $500,000 for winning the title, only teamed up a year ago but are parting ways after the tournament.
"Sania is in the prime of her career at the moment and I just said to her at the U.S. Open that I'm not too sure what I'm going to do next year and how much I'm going to play," said Black.
I think it was fair for her to tell her to find somebody. I think she's found a great partner. I wish them all the best."
Mirza, who captured her first WTA Finals title, will partner Hsieh in the future but said she was grateful to have played alongside Black.
"To end this way, we couldn't have asked for a better start or end. It kind of sums up our partnership," Mirza said.
"I've also found a great friend in her. Almost like an older sister to me. I'm the oldest in my family, so I learned a lot from her on and off the court."
It was also a bittersweet moment for Tsieh and Peng, who also played together for the last time. In addition to last year's WTA Finals, they also joined forces to win the Wimbledon women's doubles in 2013.
