 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.Guinto, after serving briefly as private secretary to Senate President Manuel Quezon, he launched his political career in 1922 by running as member of the Provincial Board of the then Tayabas Province, home province of his wife Marta Montes.By 1925, he pursued the position of representative of the 2nd district of Tayabas under the Lower House of the Philippine Legislature.