Samuel Montero was born in the city of San Pedro de Macoris on November 12, 1968. His father, Juan Bautista Montero, was an agricultural worker, and his mother, Lucrecia, cooked and delivered meals for other agricultural workers in town. Sosa was one of seven children. His parents worked very hard to make ends meet and to provide for all of them. His father died of a brain aneurysm when Sosa was seven years old. His death created a major disruption for the family, which although poor had until then been able to sustain a stable life.
After his father's death, the family moved into a spare two-room apartment located in a building that once had been a hospital. His mother worked as a housemaid for a wealthy family to support her children. Sosa and his older brother had to go and find odd jobs to help the family. He washed cars and sold fruit and agricultural produce to help, but like many other children of San Pedro, he worked primarily as a shoeshine boy.