Keith Noel Emerson was born on 2 November 1944 in Todmorden, West Riding of Yorkshire. When Emerson was eight, his parents arranged formal tuition, learning to play and read music with "local little old ladies" until he was around thirteen, with whom he studied to ABRSM Grade 7. Emerson's teacher put him in competitions at the Worthing Music Festival and suggested he finish studying music in London, but Emerson had little interest in classical music at the time and chose jazz piano. His studies in Western classical music largely inspired his own style in his professional career which often incorporated jazz and rock elements. Although Emerson did not own a record player, he enjoyed listening to music on the radio, particularly Floyd Cramer's 1961 slip note-style "On the Rebound" and the work of Dudley Moore. He used jazz sheet music from Dave Brubeck and George Shearing and learned about jazz piano from books and Andre Previn's version of My Fair Lady. He also listened to boogie-woogie, and to country-style pianists including Joe "Mr Piano" Henderson, Russ Conway and Winifred Atwell. Emerson later described himself: "I was a very serious child. I used to walk around with Beethoven sonatas under my arm. However, I was very good at avoiding being beaten up by the bullies. That was because I could also play Jerry Lee Lewis and Little Richard songs. So, they thought I was kind of cool and left me alone."

Emerson became interested in the Hammond organ after hearing jazz organist Jack McDuff perform "Rock Candy", and the Hammond became his instrument of choice in the late 1960s. Emerson acquired his first Hammond organ, an L-100 model, at the age of 15 or 16, on hire purchase and a loan from his father. He had saved money to buy a Bird electric organ with built-in speakers on each side, but then spotted a Hammond in the shop and thought it was a better purchase. Emerson's initial plan was for a non-musical career while playing the piano on the side. Upon leaving school he worked at Lloyds Bank Registrars where he played the piano in the bar at lunch times and local pubs at nights. He was ultimately fired from the bank. Emerson played in a local 20-piece swing band run by Worthing Council, performing Count Basie and Duke Ellington tunes. This led to the formation of the Keith Emerson Trio, with the group's drummer and bassist. While performing in the Worthing and Brighton area, Emerson played in John Brown's Bodies where members of The T-Bones, the backing band of blues singer Gary Farr, offered him a place in their group. After a subsequent UK and European tour with the T-Bones, the band split. Emerson then joined The V.I.P.'s, which he described as a "purist blues band"; his noted flamboyance began when a fight broke out during a performance in France. Instructed by the band to keep playing, he produced some explosion and machine gun sounds with the Hammond organ, which stopped the fight. His band members told him to repeat the stunt at the next concert, where Emerson played the organ back to front. In 1967, Emerson formed The Nice with Lee Jackson, also of the T-Bones, David O'List, and Ian Hague, after soul singer P. P. Arnold asked him to form a backing band. After replacing Hague with Brian Davison, the group set out on its own, quickly developing a strong live following. The group's sound was centred on Emerson's Hammond organ showmanship and theatrical abuse of the instrument, and their radical rearrangements of classical music themes as "symphonic rock". To increase the visual interest of his show, Emerson abused his Hammond L-100 organ by, among other things, hitting it, beating it with a whip, pushing it over, riding it across the stage like a horse, playing with it lying on top of him, and wedging knives into the keyboard. Some of these actions also produced musical sound effects: hitting the organ caused it to make explosion-like sounds, turning it over made it feed back, and the knives held down keys, thus sustaining notes. Emerson's show with The Nice has been cited as having a strong influence on heavy metal musicians. Away from The Nice, Emerson was involved in the 1969 Music from Free Creek "supersession" project that included Eric Clapton and Jeff Beck. For the session, Emerson performed with drummer Mitch Mitchell and bassist Chuck Rainey covering, among other tunes, the Eddie Harris instrumental "Freedom Jazz Dance". Emerson first heard a Moog synthesizer when a record shop owner played him Switched-On Bach (1968) by Wendy Carlos, and thought the instrument looked like "an electronic skiffle". He got into contact with keyboardist Mike Vickers, who had paid £4,000 to have one shipped from the US, and organised to play it at an upcoming The Nice concert at the Royal Festival Hall with the Royal Philharmonic Orchestra, in February and March 1970. Vickers helped patch the Moog, and the concert saw Emerson perform "Also sprach Zarathustra" by Richard Strauss with Vickers behind the machine to swap patches. After The Nice split in March 1970, Emerson formed a new band, Emerson, Lake & Palmer (ELP), with bassist Greg Lake from King Crimson and drummer Carl Palmer from Atomic Rooster. After four months of rehearsal, the band played its first shows and recorded its first album, having quickly obtained a record deal with Atlantic Records. ELP became popular immediately after their 1970 Isle of Wight Festival performance, and continued to tour regularly throughout the 1970s. Not all were impressed, with BBC Radio 1 DJ John Peel describing their Isle of Wight set as a "tragic waste of talent and electricity". Their set, with a half-million onlookers, involved "annihilating their instruments in a classical-rock blitz" and firing cannons from the stage, which had been tested out on a field near Heathrow Airport. Following ELP's 1974 tour, the members agreed to put the band on temporary hiatus and pursue individual solo projects. During this time, Emerson composed his "Piano Concerto No. 1" and recorded it with the London Philharmonic Orchestra. According to Emerson, he was motivated by critical comments suggesting that he relied upon adapting classical works because he was unable to write his own music, and further motivated by the London Philharmonic "who weren't that helpful to begin with" and "had the attitude of 'What's a rock musician doing writing a piano concerto?'" Emerson said, "I wanted people to say, look, I'm a composer, I do write my own music, and what greater challenge than to write a piano concerto." The recording later appeared on ELP's album Works Volume 1. Emerson's concerto has since been performed by classical pianists, most notably Jeffrey Biegel, who has performed it several times and recorded it with Emerson's permission.