At age 23, he ran successfully for
Mendham Borough council about a year after moving to the borough, defeating an incumbent
Republican and becoming the lone
Democrat on the council. In 1970, he was elected Mayor of the predominately Republican town and served from 1971-1974. He resigned before the end of his term after being appointed by Governor
Brendan Byrne to be deputy commissioner of New Jersey Department of Institutions and Agencies. In 1976, he was then appointed by Governor Byrne to the state's first Corrections Commission. He then spent 19 years (1978–1998) as president and CEO of the
New Jersey Sports and Exposition Authority. but he withdrew his name from the selection process. After arriving at Rutgers on April 15, 1998, Mulcahy used personal contacts to raise the Athletic Department's endowment and New Jersey political contacts to obtain funding from the New Jersey legislature for a massive renovation of Rutgers athletic facilities. He also won increased television coverage for Rutgers football. In 2008, a series of scandals hit the press with investigative reporting by the Newark
Star-Ledger leading to disclosures concerning the handling of Athletics Department finances. Mulcahy was fired in December 2008 by then-president
Richard McCormick under pressure resulting from the
Star-Ledger investigations, though he said he had done nothing wrong. Mulcahy served as the chairman of the New Jersey
Casino Reinvestment Development Authority from 2014 to June 2021. In 2012, he joined the board of trustees of
Georgian Court University. He served on the board until his death. ==Awards and honors==