Miami instituted a commissioner/manager form of city government in 1921 and the Department underwent significant changes with the new system. The Chief of Police and the Public Safety Director were to be appointed by the City Manager and the police officers were placed under civil service guidelines, which established age, physical and written examinations, and probation requirements. The City Manager appointed Mr. Leslie Quigg as the new Chief of Police. Under Chief Quigg's leadership, the Miami Police Department grew from 40 personnel to over 200 members.

Chief Quigg reorganized the Department into four divisions (traffic, detectives, vice, and motors), created a school police force and campaigned forcefully to improve the relationship between the Department and the citizenry.

During the 1930's and 1940's the Miami Police Department experienced a wide range of changes. The police radio system was established. The Police Benevolent Association was created to present a unified forum for police officers to deal with civil service issues. And in 1944, under Chief Charles Nelson, the first black police officers were trained and deployed to Miami's growing black communities. Three years later, the Civil Service Commission gave these officers civil service status. True integration into the Department would have to wait for several years.