Guinto was born to Juan P. Guinto and Pia Gawaran in the village of San Nicolas in Bacoor, Cavite province.
By 1916, Guinto left his job at the Weather Bureau to pursue law studies at the old Escuela de Derecho and by 1920, after completing his law studies and qualifying as a bona fide lawyer, Guinto was employed as private secretary to the then Philippine Senate President, Manuel L. Quezon.