In the run-up to his election and throughout his first term as mayor, Bill de Blasio built a reputation for advancing women’s rights. He was endorsed by the National Organization for Women; held a ticker-tape parade for the United States women’s soccer squad, the city’s first for a women’s team; and won points for placing many women in high-level positions.

But now, as he seeks re-election in November, Mr. de Blasio has seen a puzzling exodus of female talent.

Thirty-one high-level officials hired by Mr. de Blasio have left City Hall since he took office in 2014. Of those, 22 were women, more than twice the number of men who left.

The gender imbalance of the departures has raised questions as to why so many women, including many who are black or Hispanic, have jumped ship on a mayor seen as welcoming to women and minorities.