Marvin Laird, a conductor for Broadway musicals and for performers like Bernadette Peters who also composed the music for “Ruthless!,” the campy, award-winning Off Broadway show about a girl who will do anything — including kill — to star in a school play, died in a hospital on Dec. 2 in Bridgeport, Conn. He was 85.

His partner in marriage, Joel Paley, said his death, in a hospital, was caused by complications of an infection.

Mr. Laird was the assistant musical director for a summer stock production of “Gypsy” in Lambertville, N.J., in 1961 when he met Ms. Peters, who was 13 and was playing two small roles.

“He was just the most energetic, charismatic fellow you’d ever want to meet,” Ms. Peters said in a phone interview.

He later conducted the orchestras for her concerts and for two Broadway revivals in which she starred: “Annie Get Your Gun” in 1999 and “Gypsy” in 2003. When Ms. Peters appeared in a revival of “Follies” in 2011, he was the associate conductor.

“The orchestras loved him,” Ms. Peters said. “He had a great sense of humor and they respected his musicianship.” She added: “He knew what I was going to do before I did it. I don’t sing a song the same way twice; it’s whatever happens to the song. And Marvin could get the whole orchestra to breathe with him.”