The United States-Czech pair tamed unseeded Australians Ashleigh Barty and Casey Dellacqua 6-2, 6-1 in the final.
Without any double faults or unforced errors, the duo of Mattek-Sands and Safarova fared stable to break six times to seal the win in just one hour and six minutes, reports Xinhua news agency.
"We played really well today. We played a good team. We watched them play their semi-final match. They beat another good team in three sets, so we were ready with what they were going to bring out on the court," said Bethanie.
"We are taking it one match at a time. We enjoy our practice days and off-days," she said.
"We're really excited for grass. We're looking forward to Wimbledon. We love playing on grass, and we are going to be ready there too."