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 Senate President, Manuel L. Quezon.

He was elected provincial governor of Tayabas in 1928 but his term was cut short when he was appointed as Commissioner of Public Safety by then American Governor-General Theodore Roosevelt Jr.

By 1942, Guinto was appointed by Jorge B. Vargas, the then incoming chairman of the Japanese-created government structure called the Philippine Executive Commission, to assume the position of mayor of City of Greater Manila and look after the city's administration during the Japanese occupation during World War II.

A blanket amnesty was issued before the granting of Philippine Independence on July 4, 1946, sparing the former Manila mayor of a war crimes trial.

Guinto went into the private sector, taught in the academe and even served as Dean of the College of Arts and Sciences of the Lyceum of the Philippines University.