Years after the Mafia was all but declared dead, federal agents conducted their largest ever roundup of suspected mob members and associates Thursday, charging 127 people in the Northeast with crimes ranging from execution-style slayings to illegal card games.

Early-morning raids in New York, New Jersey and Rhode Island netted 119 people, charged in 16 separate indictments, that authorities say are affiliated with La Cosa Nostra. Five of those charged were in prison; three...