Stephen C. Robinson, the new head of the New York City Bar’s diversity committee, said he had been on the Southern District federal bench for four years before the realization hit him—he had never hired a black clerk.

By the time of his awakening, Mr. Robinson, who was the first black U.S. Attorney for the District of Connecticut, had hired eight clerks, seven of whom were white and one of whom was Indian. Out of the thousands of resumes Mr. Robinson received each year, he said, he had interviewed only two blacks up to that point.