Erin Patrick, a deputy medical examiner in Multnomah County, said Mitchell was found dead shortly after 3 a.m. Wednesday in his room at the Benson Hotel in downtown Portland. She said Mitchell apparently died of natural causes. An autopsy is planned.
Hendrix, one of music's most celebrated guitarists, joined with Mitchell and bass player Noel Redding to create the Jimi Hendrix Experience in London in 1966.
The following year, the band released its debut album, "Are You Experienced?" featuring the tracks "Hey Joe" and “Purple Haze.”
The group dissolved shortly before Hendrix's death in 1970 at age 27. Redding died in 2003.
Mitchell had been performing with a Hendrix tribute band that gave a concert Friday in Portland. It was the last stop on the West Coast portion of the tour, which played Nov. 2 at the Greek Theatre in Los Angeles.
The Jimi Hendrix Experience made a lasting mark on music during its three-year run. In a span of 18 months, the group recorded "Axis: Bold as Love" and "Electric Ladyland," as well as "Are You Experienced?" All three were "landmark albums," according to the Rock and Roll Hall of Fame.
In addition, Hendrix's "theatrical, incendiary performances at the Monterey Pop and Woodstock festivals, including the ceremonial torching of his guitar at Monterey, have become part of rock and roll legend," the Hall of Fame says in an online tribute to the group, which was inducted to the hall in 1992.
Mitchell played with Hendrix at Woodstock after the Experience broke up. Along with bassist Billy Cox, he also worked with the guitarist on 1970's "Cry of Love" album.
"Mitchell’s drumming, as instinctive as his leader's guitarwork, was a perfect foil," the Encyclopedia of Popular Music says in its listing for the Jimi Hendrix Experience.
John "Mitch" Mitchell was born July 9, 1947, in England. Before he joined Hendrix's trio in 1966, he played drums with English bands that specialized in R&B covers. In later years, he had been a session musician for various rock bands.
In a 1990 review of Mitchell's coffee-table book "Inside the Experience," the Times of London described him as "a feisty character with a small, tough build of a jockey, and a polished manner that belies a wary disposition."
Reviewer David Sinclair went on to note that Hendrix had in Mitchell a "uniquely imaginative drummer, whose playing contributed greatly to Hendrix's sound."