Prince Tu'ipelehake, 56, Princess Kaimana, 46, and their driver died on Wednesday night, according to Senter Uhilamoelangi, a distant relative and long time friend of the prince.
The 18-year-old driver who hit the prince's sport utility vehicle (SUV) was travelling as fast as 100 mph (160 kph) in Menlo Park, about 48kms (30 miles) south of San Francisco, highway patrol Officer Ricky Franklin said.
The woman survived the crash and faced arraignment on Friday.
Tu'ipelehake, a nephew of King Taufa'ahau Tupou IV, who led a national committee studying democratic reforms, owned a mansion in Hillsborough, about 24kms (15 miles) south of San Francisco.
He and his wife had come to the area earlier this week to discuss politics with members of the region's Tongan community, said Uhilamoelangi, who lives in East Palo Alto and helped arrange the visit.
'Akilisi Pohiva, a democracy movement leader in the Tongan capital of Nuku'alofa, called the deaths " a great loss to the country."
"People are very much concerned, especially the people who are pushing for change," Pohiva said.
The deaths have stunned the Tongan community, which numbers about 37,000 in the United States, including 15,000 in California, according to the 2000 Census.
The prince was scheduled to speak on Thursday at a Tongan church in San Bruno, and the event became a memorial service.
Now the last monarchy in the Pacific, Tonga has been a Polynesian kingdom and was a protectorate of Britain until it acquired independence in 1970.
The 170-island archipelago, about halfway between Australia and Tahiti, has a population of about 108,000 and an economy dependent on pumpkin and vanilla exports, fishing, foreign aid and remittances from Tongans abroad.