Barry died Sunday at the United Medical Centre in the US capital shortly after midnight and no cause of death has been announced, although he had suffered from multiple and serious health problems in recent years.
Considered to be the most influential municipal politician in Washington in the late 20th century, Barry had remained active as a member of the District of Columbia city council, a post he had occupied for the past 15 years.
Barry, who was black, was mayor of Washington from 1979 to 1991 and again later between 1995 and 1999.
Originally from the state of Mississippi, Barry began his political career as a civil rights activist during the 1960s and 1970s.
Nevertheless, and despite his enormous popularity, he got into highly publicised trouble for his ongoing problems with drugs and alcohol, his extramarital affairs and his attempts to avoid paying his fair share of taxes.
In 1990, he became nationally and internationally known when he was arrested in an FBI operation for smoking crack cocaine in a capital hotel with one of his love interests.
After serving a term in prison and going through a rehab programme, the charismatic politician returned to the public arena for another term as mayor in the late 1990s.
Barry considered himself to be the defender of lower-income citizens and he is still enormously popular in the capital's most disadvantaged neighbourhoods, including Anacostia, which has a black majority and whose district he represented.
During his terms as mayor, he appointed African Americans to many local government posts, especially offices that traditionally had been reserved for whites.
Last June, Barry published his autobiography entitled "Mayor for Life: The Incredible Story of Marion Barry, Jr".
The Washington Post, with which Barry had more than one confrontation during the course of his career, Sunday called him "the most influential and savvy local politician of his generation," adding that "he dominated the city's political landscape in the final quarter of the 20th century".