In such grim circumstances, a macabre sense of humor flourished in Ms. Holdman. In 1981, on the anniversary of Florida’s first execution under the new laws, she sent Florida’s attorney general at the time, Jim Smith, a “deathday cake” with black candles.

She later worked at the California Appellate Project in San Francisco and at the Center for Capital Assistance in New Orleans, where she trained lawyers and investigators how to develop evidence that could be used to secure a life sentence.

“What she saw is that killers are not just born,” said George Kendall, a lawyer who represents death row inmates. “They have had unbelievably abused and neglectful lives, and that history is relevant. You become your client’s biographer, you speak to the 60 most important people in that person’s life — friend and foe.”

Many clients had suffered sexual abuse and other traumas, and trust was critical. “How do you get people to talk about the worst family secrets?” asked James Lohman, a lawyer who worked with her in Florida. He added: “None of that comes easily. She figured it out and then trained people how to do it.”

Many mitigation specialists who followed in her footsteps are journalists and social workers. “It’s the antithesis of being a lawyer; it’s all about human feeling and connection,” Mr. Lohman said.

In recent years, Ms. Holdman worked with the lawyer Judy Clarke on the cases of Jared Loughner, who pleaded guilty to the 2011 mass shooting in Tucson in which Gabrielle Giffords, then a congresswoman, was seriously wounded, and of Eric Rudolph, who was responsible for the 1996 bombing at the Summer Olympics in Atlanta that left one dead and more than 100 injured. Armed with Ms. Holdman’s voluminous research of both men’s lives, their defense teams worked with prosecutors to let them plead guilty in exchange for life sentences.

Ms. Holdman was famous for her devotion to her clients, and they often grew attached to her; after Mr. Kaczynski was sentenced to life in prison for a series of bombings, he asked that she be given his Montana cabin. (The government did not let her keep it, according to The New Yorker.)