Jennifer Diaz first stepped onto the stage of the Metropolitan Opera House at 11, as an extra in “I Pagliacci” and “La Bohème.” For the next three years, it became her second home, as she regularly visited her father, who is still the Met’s night crew manager after more than three decades.

At 19, she began working there as a stagehand, typically the only female member in a 28-person crew.

Now, at 34, she has made history, becoming the first female head carpenter of Local 1 of the International Alliance of Theatrical Stage Employees. The local’s 3,351 members work in spaces from the Met to Carnegie Hall, at Radio City Music Hall and Madison Square Garden, and in every Broadway theater — including the Walter Kerr, which is where she was one morning in September, overseeing the load-in for the musical “Falsettos.”

With a head of thick dark curls and a ready smile, Ms. Diaz is a self-described tomboy, a blend of low-key authority and quiet confidence. “My name has a lot of clout in this business,” she said. “I have people on my side and in my pocket I can turn to.”