The scribes will be honored at an event to coincide with the bicentennial of Edgar Allan Poe's birth next year.
The organization said this is the first time since 1978 that it has presented dual Grand Masters.
MWA said the award "represents the pinnacle of achievement in the mystery genre and was established to acknowledge important contributions to the genre, as well as significant output of consistently high-quality material."
The awards will be presented at the 63rd annual Edgar Awards banquet April 30 at the Grand Hyatt Hotel in New York.
"One of the great pleasures of my tenure at the helm of MWA has been informing two of the most talented writers on the planet that they have been selected as Grand Master," MWA executive vice president Harry Hunsicker said in a statement. "As a long-time fan of both, I cannot think of two more deserving individuals."
"News of my being named MWA Grand Master, along with James Lee Burke, caught me totally off-guard," Grafton said. "But I'm delighted to be tapped and pleased to share the honor with a writer I so admire."
"It is a great honor to receive the Grand Master award from the Mystery Writers of America. It's an enormous compliment to my work and to me, and I feel extremely grateful and humbled by the MWA's generosity of spirit," Burke said.