Bill de Blasio, the former mayor of New York City, accepted blame and agreed to pay $329,000 to the city to resolve a conflicts-of-interest violation for bringing his security detail on trips during his failed presidential campaign in 2019.

The Conflicts of Interest Board, which announced the settlement on Wednesday, had found that Mr. de Blasio violated the law by billing the city for his security detail to travel with him and his wife on 31 out-of-state trips, even after the board advised Mr. de Blasio in advance that the expenses were not allowed.

The settlement, which reduced the board’s fine from $155,000 to $10,000, was the board’s first enforcement action against a mayor. The board said that Mr. de Blasio had already paid $100,000 of what he owed and would pay the rest in quarterly installments over the next four years.