Lee Mendelson -- the man behind over 50 Charlie Brown TV specials -- died on Christmas Day after a long battle with cancer ... according to his family.

The esteemed "Peanuts" producer reportedly passed away at his home in Hillsborough, CA. His son, Jason, says ... "It wasn’t great for us, but to have him pass on Christmas really ties into his history and legacy."

Along with producing the holiday television classic, "A Charlie Brown Christmas" in 1965, Lee also wrote the lyrics to “Christmas Time Is Here” ... the song famously featured in the special. The special brought Mendelson the first of his 12 Emmy Awards.

Mendelson earned his 12th Emmy in 2015 for “It’s Your 50th Christmas, Charlie Brown.” He's also won 4 Peabody Awards and was nominated for an Oscar and 2 Grammys.

Lee's relationship with "Peanuts" creator Charles Schulz began in the '60s on the heels of Mendelson's acclaimed documentary about Willie Mays. Schultz saw it and agreed to work with Lee on another documentary ... but ended up producing the 'Charlie Brown Christmas' special instead.

The success of that led to 1966’s “It’s the Great Pumpkin, Charlie Brown” and 1973’s “A Charlie Brown Thanksgiving,” among many others.

Mendelson teamed with "Garfield" creator Jim Davis in the '80s to produce the first animated short featuring the grumpy cat ... and went on to help launch the "Garfield and Friends" series in the early '90s.

Lee produced several documentaries and network specials outside of children's programming, as well. He's survived by his wife, Ploenta, 4 children, a stepson and 8 grandchildren.

Mendelson was 86.