 By 1934, the Departments of Interior and Labor were merged and Guinto continued to serve as undersecretary.In 1940, Guinto was appointed Secretary of Labor in the Commonwealth government of President Manuel L. Quezon.By 1942, Guinto was appointed by Jorge B. Vargas, (Chairman of the Japanese created government structure called the Philippine Executive Commission) to assume the position mayor of Greater Manila, and look after the city's administration during the Japanese occupation during the Second World War.