Cervantes' political career began in 1949 when he was elected to the St. Louis Board of Aldermen representing the 15th Ward.

In 1965, Cervantes challenged incumbent Mayor Raymond Tucker, who was seeking an unprecedented fourth term in office.

During his term as mayor, Cervantes focused in particular on the issues of race relations, crime-fighting, and city finance.

Mayor Cervantes was successful in getting a $2,000,000 bond issue passed for completion of the Gateway Arch and grounds.

Other successes during his two terms included establishment of Night Housing Court, organization of a Business Development Commission to help keep businesses in the City and bring in new ones along with setting up the Area Office of Aging, Beautification Commission, and Citizens Service Bureau to handle complaints.

Mayor Cervantes failed in his attempt to get a new airport started in Illinois, close to downtown St. Louis.