The 17-year-old Craigielaw player, who also visited Arizona State, Georgia Tech, Duke and East Tennessee State during an exploratory trip to the States earlier this year, has signed a letter of intent to enrol at San Diego next September.
Forrest, who won the Scottish under-16s title at Largs in 2009 before adding the under-18 crown at nearby West Kilbride in April, consulted James Byrne, who is in his final year there, about Arizona State and was also aware that two other Scottish internationals, Michael Stewart and Jordan Findlay, had spells at East Tennessee State. However, he has opted for San Diego on the strength of a number of factors, which include the fact Mickelson, who played in the Amateur Championship at Muirfield and North Berwick this year, has transformed the 'Toreros' from also-rans into one of the top teams in college golf.
"It was a tough decision. In fact, I probably had too many to choose from. However, I've gone for the University of San Diego due to the fact it has a very good golf programme, the weather there is excellent and the education is good as well," Forrest told The Scotsman. "Tim Mickelson has done a great job there.
"I believe he's been at San Diego for the last eight years, during which time they've gone from being ranked 270th in the college rankings into the top 25. That is very impressive."
Mickelson believes he has pulled off a coup by signing Forrest, who added a Great Britain & Ireland Boys' cap to his CV this year in the Jacques Leglise Trophy match against Continental Europe in Italy.
"We are very excited that Grant will be joining the USD golf family next September," said the USD head coach. "Grant is a very accomplished player who will make an immediate impact for us next year.
"He has already proven his talents on the course and he is also a true gentleman. From the time I met him on his official visit, I knew Grant would be an excellent fit at USD. He is also an excellent student who takes pride in his academics. We could not be more excited to embrace him into our programme next fall."
Forrest, who left Stewart's Melville College at the end of fifth year, is eligible to defend his Scottish Boys' title at Dunbar in April.He is also hoping to make his mark in some of the bigger men's events as well next season before heading off to start his four-year scholarship in Southern California.
"I've given myself a hard act to follow after this season but I definitely think there is room for improvement next year and I'm working in the gym at the moment with a view to being ready for the start of the new campaign," he said.
Forrest will join a number of fellow Scots on the US college circuit when he starts his spell at San Diego.
They include Pollok's Conor O'Neil, this season's Scottish Boys' Order of Merit winner, who recently joined Edinburgh's James Ross at Michigan State.