Dinkins, the city's first black mayor, was elected in 1989 and lost his re-election bid to Rudolph Giuliani (search) four years later.
Dinkins was to announce the endorsement in an afternoon conference call with reporters Wednesday.
On Monday, Dinkins attended a Martin Luther King Jr. Day event with one of Clark's opponents, the Rev. Al Sharpton, whom he called "the people's minister."
During his remarks at the King event, Dinkins said as long as an administration picks people like "arch conservative Charles Pickering," the Mississippi federal judge recently appointed to an appeals court, "justice will be denied and it will be our job to choose other powers to make those choices."
A crucial first test of the Democratic contest in the South is the Feb. 3 South Carolina primary, where black voters are expected to make up as much as half of the electorate.
Clark has campaigned in the South with former Atlanta mayor and U.S. ambassador Andrew Young, and Rep. Charles Rangel, a New York congressman who voiced his support for Clark at the beginning of his campaign.
Rangel, a Korean war veteran, has worked behind the scenes in his home district of Harlem, around the state and in the Congressional Black Caucus on Capitol Hill to win support for Clark, based largely on Clark's opposition to the war in Iraq.