In 1955, he ran as the Democratic nominee for Mayor of Little Rock and defeated two term incumbent Pratt C. Remmel, a Republican. He took office in January 1956 and immediately set about to make a lot of changes. In addition to revitalizing the City’s bus system, and removing some color barriers at City Hall, he oversaw the dismantling of the copper dome on top of Little Rock City Hall (as opposed to the repair of the dome championed by Mayor Remmel).
Mayor Mann was caught up in a grand jury investigation into purchasing practices at City Hall as well as within the City government in North Little Rock. Partially in response to this, Little Rock voters approved a new form of government in late 1956. Mayor Mann opposed the switch to the City Manager form and refused to set the election for the new officials but was ultimately compelled to do so.
He was also Mayor during the 1957 integration of Little Rock Central High School. He sought to keep the peace and to broker a deal between President Dwight Eisenhower and Governor Orval Faubus. His powers within the city were, no doubt, hampered because of his lame duck status as Mayor. In November 1957 following the election of the new City Board of Directors, he chaired his last City Council meeting and left office.
In January of 1958, a series of articles written by Mayor Mann detailed his perspective on the events at Central High. These were carried by newspapers throughout the US.
Because of ill will toward him due to the Central High crisis (he was criticized by both sides) and grand jury investigation, Mayor Mann felt it would be difficult to maintain his insurance business in Little Rock. He moved to Texas in 1959 and remained there the rest of his life. He died in Houston on August 6, 2002.