After a rocky 100 days as chancellor, Cathie Black is leaving the job today, Mayor Bloomberg is set to announce in minutes at a press conference at City Hall.

Black is being replaced by Dennis Walcott, a deputy mayor who has long played a prominent role in education issues. At Black’s public appearances during most of her brief tenure as chancellor, Walcott could almost always be found at her side, managing her interactions with the press and others. Walcott is a graduate of the city’s public schools.

The leadership change came as a surprise as Bloomberg had as recently as yesterday defended Black against low public opinion. But the departure of two top deputies this week dealt a crucial blow to her leadership.

A Marist poll earlier this week found that just 17 percent of New Yorkers approved of Black’s job performance. The lowest approval rating ever posted by her predecessor, Joel Klein, who announced his resignation in November, was 33 percent. A different poll last month found that 27 percent of New Yorkers approved of Bloomberg’s school policies, down from 54 percent 18 months earlier.