His body was found at Vena Avenue Elementary School by a childhood friend about six hours after Lehman, 20, told the friend he wanted to kill himself, Los Angeles police said.
Foothill Division detectives said Lehman apparently took his life because he was despondent over breaking up with his girlfriend and losing money in an unreported burglary of his Lankershim Boulevard apartment in North Hollywood.
His friend, Joseph Allen, also 20, told police he returned from an evening out at 1:45 a.m. to discover Lehman's body hanging from a belt looped through a chain-link fence behind the school, 9377 Vena Ave.
A note was found in Lehman's pocket indicating he wanted what remained of his belongings to go to his mother, who lives with his stepfather in Colorado Springs, Colo., Foothill investigators said.
Lehman, who was raised in Arleta, had recently been unemployed and apparently had little or no acting work since "Nanny and the Professor" went off the air 11 years ago, said Detective Albert A. Ferrand.
Lehman portrayed 8-year-old Butch Everett, one of the professor's three children, in the 1970-71 series on ABC, a network spokesman said.
His father, who now lives in Carson City, Nev., said Lehman was recently interested in carpentry work.
Allen, who lives across the street from the school, said he had told Lehman Sunday night he was welcome to spend the night in Allen's car.
That was after Lehman asked Allen for a gun, saying he wanted to kill himself because a long relationship with his girlfriend had ended and because he lost a large amount of money in the recent burglary of his apartment, police said.
Allen did not give Lehman a gun, Ferrand said.
The detective said he was trying to find Lehman's girlfriend to interview her.
Ferrand said he would also like to find out why Lehman had not reported the burglary to police.