The British musician's demise was confirmed by his band in a statement, reported BBC.
"We regret to announce that Keith Emerson died last night at his home in Santa Monica, Los Angeles," read a statement on the band's Facebook page.
He died of a self-inflicted gunshot wound to the head, Santa Monica police confirmed to the BBC. His death was being investigated as a suicide, police added.
A police spokesman said Emerson's body was found in the early hours of Friday morning by his girlfriend Mari Kawaguchi at their flat in the Californian city.
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"Keith was a gentle soul whose love for music and passion for his performance as a keyboard player will remain unmatched for many years to come."
Born in Yorkshire, England, in 1944, Emerson was considered one of the top keyboard players of the progressive rock era.
Inspired by Jimi Hendrix's theatrics with the electric guitar, Emerson was famous for his showmanship and outlandish on-stage performance style.
ELP achieved an international following and were particularly popular in Britain and Japan.
Before ELP, Emerson was a member of The Nice, which was formed in 1967 but disbanded three years later.
In later life, he pursued a solo career and remained active in the music business.
He was forced to call off a tour in 2010 due to abnormal growth in his colon, but had a tour of Japan scheduled for next month.
His last concert took place in July 2015 at the Barbican in London, where he performed alongside the BBC Concert Orchestra in a tribute to Robert Moog, the inventor of the Moog synthesizer.