Steve Harwell, a founding member and former leader of the US rock band Smash Mouth, known for its hit songs All Star and Walkin' on the Sun, died Monday at age 56, his agent said.

Harwell, who left the group in 2021 for health reasons and had been in hospice care in recent days, died at his home in Boise, Idaho "surrounded by family and friends," Smash Mouth manager Robert Hayes told Rolling Stone magazine.

Hayes said the cause of death was liver failure.

Harwell was a singer who founded Smash Mouth in California in 1994 along with Kevin Coleman on drums, Greg Camp on guitar, and Paul De Lisle playing bass.

The group is still active and its style is described as a mix of pop, rock, ska and punk.

Smash Mouth shot to worldwide fame in 1997 with its album Fush Yu Mang and the hit song Walkin' on the Sun.

"Walkin' on the Sun changed music," Harwell told Rolling Stone in 2019. "It changed the way people listen to music."

"It was so different and it was so unusual, and it was so special," he said. "It just had that sound that we created. Ask anybody that's tried to copy us, you can't. You just can't."

Two years later came the hit All Star on the album Astro Lounge, a tune that was nominated for a Grammy and features on the soundtrack of the first Shrek movie - about a loveable green ogre voiced by Mike Myers.

More than 25 cover versions of All Star have been done over the years, a sign of its popularity, and it has been streamed on Spotify nearly a billion times.

In a statement Hayes paid tribute to Harwell, saying "Steve's iconic voice is one of the most recognizable voices from his generation."

With Harwell as lead singer Smash Mouth sold 10 million records around the world, Hayes said.

Since the 2010s, Harwell suffered several health scares either in concert or while rehearsing and cited heart trouble when he left the group.

"Steve lived a 100 per cent full-throttle life. Burning brightly across the universe before burning out," Hayes said.