Variety reports that the cause of death was cancer. He was 86. Mendelson's son, Jason, told TMZ, "It wasn't great for us, but to have him pass on Christmas really ties into his history and legacy."
Mendelson wrote the lyrics to "Christmas Time is Here," featured in "A Charlie Brown Christmas." Over his career, he won 12 Emmys, four Peabody Awards, an Oscar nomination and two Grammy nominations, Variety reported.