Mighten is English-American, and represented the Three Lions at the youth level up until the U20 level. He also has USMNT eligibility.
As a professional, the 22-year-old has 103 senior appearances with six goals and two assists across loans to Sheffield Wednesday and Port Vale in England, a loan stint with KV Kortrijk in Belgium and his time with Forest.
He becomes SDFC's sixth-ever signing, following the blockbuster purchase of Mexico star Chucky Lozano earlier this summer.
"I'm excited to be joining San Diego FC at such an exciting time for the club," said Mighten. "The opportunity to develop further at SDFC and MLS is a perfect scenario for my growth as a player. I can't wait to contribute to the club's success and help build something special in San Diego."