San Martin was born in Spain on the island of Mallorca. When he was five years old, his family moved to Montana, where he was first stung by the acting bug and was a regular in children's theatre productions. He was raised there until he was in his early teens and later returned to Spain with his family to live in Madrid, where he worked as a model. While considering enrolling in art school to become a sculptor, Francisco started taking acting lessons in Madrid. He pursued acting opportunities and eventually appeared in various stage, television and feature film productions in Spain.
To further his professional training as an actor, San Martin moved to Los Angeles in 2010, and studied with The Acting Corps. He currently studies with Sal Landi at The Actorhood Theatre Company. A one-day appearance on Days brought San Martin to the attention of the producers, and this led to a series regular role.