Unbeaten Garcia, 21, caught Fonseca with a left hook to the head to send him to the canvas and secure his first-round victory in Anaheim.
"I was planning for it to go longer," Garcia told DAZN after the fight.
Garcia says he is now targeting fights against Jorge Linares, Luke Campbell, Gervonta Davis and Devin Haney.
It was Garcia's second first-round knockout in a row, having beaten Romero Duno for the World Boxing Council Silver Light title in November.
He was joined in the ring after his fight on Friday night by American Haney, the WBC lightweight champion in recess, who is recovering from a shoulder injury.
Earlier in the night in the co-main event, Venezuela's three-weight champion Linares beat Carlos Morales with a fourth round knockout.
Promoter Oscar de la Hoya says he wants Garcia to fight Linares in July at the Staples Center in Los Angeles.
"I said if he [Linares] had a good performance I will fight him," said Garcia, who has managed 17 knockouts among his 20 wins.
"I had a plan for 2020 that was to shock the world. I am about to shock the world, I will beat Linares, beat Luke Campbell, beat Gervonta Davis and then go after Devin."