Jerry Nelson, who also brought movement and voice to puppet characters on "Fraggle Rock" and "The Muppet Show," was 78.
No cause of death was reported but Nelson had suffered from emphysema, the Los Angeles Times said.
Nelson joined with Muppets creator Jim Henson in 1965 when Henson's piano-playing Muppet Rowlf the Dog -- the first Muppet "star" -- played duets with country-music singer Jimmy Dean on ABC's "The Jimmy Dean Show."
Nelson joined "Sesame Street" in 1970, its second year, and will be "remembered for the artistry in his puppetry, his music, and the laughter he brought to children worldwide through his portrayal of Count von Count, Herry Monster, Fat Blue, Sherlock Hemlock, the Amazing Mumford and many other beloved characters," said program producer Sesame Workshop, formerly known as the Children's Television Workshop.
Nelson retired from physical puppeteering in 2004, but continued to lend his voice to several Muppets right up until his death.
He was born in Tulsa, Okla., July 10, 1934, and raised in Washington.
Nelson wrote an essay on the 40th anniversary of "Sesame Street" saying he'd "wandered the paths of least resistance" in a lifetime of "breathtaking up and down melody."
"I am either blessed or one of the luckiest bozos walking this planet or both," he wrote.
Nelson is survived by his wife, Jan. Their daughter, Christine, died of cystic fibrosis in 1982.
She had a cameo in "The Great Muppet Caper." Henson gave her a speaking part so she would become a member of the Screen Actors Guild.