The 31-year-old fast bowler, who is currently part of Australia's squad at the T20 World Cup co-hosted in the United States and Caribbean, joins a franchise which also includes New Zealand's Matt Henry and Pakistan's Haris Rauf.
Major League Cricket's second season will begin in July following the T20 World Cup.
The six-team league played its inaugural season in 2023 with New York winning the first title.
"I'm very excited to join the Unicorns ahead of the 2024 season," said Cummins. "MLC is developing at a rapid rate, and the US market potential is huge for cricket."
Cummins said San Francisco's ownership group -- the team is co-owned by Silicon Valley entrepreneurs Anand Rajamaran and Venky Harinarayan -- also played a part in his decision.
“While cricket is integral to my involvement, the global network and long-term possibilities offered by the owners specifically, and Silicon Valley more broadly, presented a unique opportunity for me and my life beyond cricket," Cummins said.
Cummins arrival comes after Major League Cricket was recently granted "List A" status by world cricket's governing body, the International Cricket Council, meaning records from the league will be incorporated into official T20 stats.
