Deep Purple, 1968–1976

It is in this period that Lord's trademark keyboard sound emerged. Ignoring the emergence of the Moog synthesizer as pioneered in rock by players like Keith Emerson, he began experimenting with a keyboard sound centred on the Hammond organ but heavier than a blues sound and often featuring distortion.[4] This delivered a rhythmic foundation to complement Blackmore's speed and virtuosity on lead guitar. Lord also loved the sound of an RMI 368 Electra-Piano and Harpsichord, which he used on songs like "Demon's Eye", and "Space Truckin'". In 1973, Lord's original Hammond C3 gave out, and he purchased another from Christine McVie of Fleetwood Mac. Around this same time, Lord and his keyboard technician, Mike Phillips, combined his Hammond C3 Organ with the RMI.[4] (Lord kept this particular Hammond C3 until his retirement from the band in 2002.)

Lord pushed the Hammond-Leslie sound through Marshall amplification, creating a growling, heavy, mechanical sound that gave a rhythmic counterpoint to Blackmore's lead playing. It also allowed Lord to compete with Blackmore as a soloist, with an organ that sounded as heavy as a lead guitar. Said one reviewer, "many have tried to imitate [Lord's] style, and all failed."[5] Said Lord himself, "There's a way of playing a Hammond [that's] different. A lot of people make the mistake of thinking that you can play a Hammond with a piano technique. Well, you can, but it sounds like you are playing a Hammond with a piano technique. Really, you have to learn how to play an organ. It's a legato technique; it’s a technique to achieve legato on a non-legato instrument."[6]

In early Deep Purple recordings, Lord had appeared to be the leader of the band, though it never made chart success in the UK until the Concerto for Group and Orchestra album (1970). For example, the band's first hit song, a cover of Joe South's "Hush", features an extended organ solo and no guitar solo. Later, Lord's willingness to play many of the key rhythm parts gave the guitarist the freedom to let loose both live and on record.

On Deep Purple's second and third albums, Lord began indulging his ambition to fuse rock with classical music. An early example of this is the song "Anthem" from the album The Book of Taliesyn (1968), but a more prominent example is the song "April" from the band's self-titled third album (1969). The song is recorded in three parts: (1) Lord and Blackmore only, on keyboards and acoustic guitar, respectively; (2) an orchestral arrangement complete with strings; and (3) the full rock band with vocals. This enhanced Lord's reputation among fellow musicians, but caused tension within the group.

Blackmore agreed to go along with Lord's experimentation, provided he was given his head on the next band album.[5] The resulting Concerto For Group and Orchestra (in 1969) was one of rock's earliest attempts to fuse two distinct musical idioms. Performed live at the Royal Albert Hall on 24 September 1969 (with new band members Ian Gillan and Roger Glover, Evans and Simper having been fired), recorded by the BBC and later released as an album, the Concerto gave Deep Purple their first highly-publicised taste of mainstream fame and gave Lord the confidence to believe that his experiment and his compositional skill had a future. The Concerto also gave Lord the chance to work with established classical figures, like conductor Malcolm Arnold (knighted in 1993), who brought his technical skills to bear by helping Lord score the work and to protect him from the inevitable disdain of the older members of the orchestra.

Classical dalliance over, Purple began work on In Rock, released by their new label Harvest in 1970 and now recognised as one of hard rock's key early works. Lord and Blackmore competed to out-dazzle each other, often in classical-style, midsection 'call and answer' improvisation (on tracks like "Speed King"), something they employed to great effect live. Similarly, "Child in Time" features Lord's playing to maximum tonal effect. The organ riff on "Child in Time" was written by Lord,[7] although it is similar to It's a Beautiful Day's 1969 psychedelic hit song "Bombay Calling".[8] Lord's experimental solo on "Hard Lovin' Man" (complete with police-siren interpolation) on the album is his personal favourite among his Deep Purple studio performances.

Deep Purple released a sequence of albums between 1971's Fireball and 1975's Come Taste the Band. Gillan and Glover left in 1973 and Blackmore in 1975, and the band disintegrated in 1976. The highlights of Lord's Purple work in the period include the 1972 album Machine Head (featuring his rhythmic underpinnings on "Smoke on the Water" and "Space Truckin'", plus the organ solos on "Highway Star" and "Lazy"), the sonic bombast of the Made in Japan live album (1972), an extended, effect-laden solo on "Rat Bat Blue" from the Who Do We Think We Are album (1973), and his overall playing on the Burn album from 1974.

Roger Glover later described Lord as a true "Zen-archer soloist", someone whose best keyboard improvisation often came at the first attempt. Lord's strict reliance on the Hammond C3 organ sound, as opposed to the synthesizer experimentation of his contemporaries, places him firmly in the jazz-blues category as a band musician and far from the progressive-rock sound of Keith Emerson and Rick Wakeman. Lord himself would rarely venture into the synthesizer territory on Purple albums, often limiting his experimentation to the use of the ring modulator with the Hammond, to give live performances on tracks like Space Truckin' a distinctive 'spacey' sound. Rare instances of his Deep Purple synthesizer use (later including the MiniMoog and other Moog synthesizers) include "'A' 200", the final track from Burn, and "Love Child" on the Come Taste the Band album.

In early 1973 Lord stated: “We're as valid as anything by Beethoven.”