"[11] At Abingdon, the Greenwood brothers formed a band, On a Friday, with the singer Thom Yorke, the guitarist Ed O'Brien and the drummer Philip Selway.

[13] He was previously in another band, Illiterate Hands, with Matt Hawksworth, Simon Newton, Ben Kendrick, Nigel Powell and Yorke's brother, Andy.

"[17] According to Selway, at On a Friday's first gig, in Oxford’s Jericho Tavern, Greenwood sat on the stage with a harmonica, “waiting for his big moment to arrive".

[20] He enrolled at Oxford Brookes University to study psychology and music, but left after his first term after On a Friday signed a record contract deal with EMI.

[29] For "Climbing up the Walls", Greenwood wrote a part for 16 stringed instruments playing quarter tones apart, inspired by the Polish composer Krzysztof Penderecki.

[32] Radiohead's albums Kid A (2000) and Amnesiac (2001) marked a dramatic change in sound, incorporating influences from electronica, classical music, jazz and krautrock.

[33] Greenwood employed a modular synthesiser to build the drum machine rhythm of "Idioteque",[34][35] and played ondes Martenot, an early electronic instrument similar to a theremin, on several tracks.

[34] According to Radiohead's producer, Nigel Godrich, when the string players saw Greenwood's score "they all just sort of burst into giggles, because they couldn't do what he'd written, because it was impossible—or impossible for them, anyway".

[41] For Radiohead's sixth album, Hail to the Thief (2003), Greenwood began using the music programming language Max to sample and manipulate the band's playing.

[48] For the BBC, Greenwood wrote "Popcorn Superhet Receiver" (2005), inspired by radio static and the elaborate, dissonant tone clusters of Penderecki's Threnody to the Victims of Hiroshima (1960).

[49] For the 2005 film Harry Potter and the Goblet of Fire, Greenwood and the Radiohead drummer, Philip Selway, appeared as the wizard rock band Weird Sisters alongside Jarvis Cocker, Steve Mackey, Steven Claydon and Jason Buckle.

[57] It features mostly 70s roots and dub tracks from artists including Lee "Scratch" Perry, Joe Gibbs and Linval Thompson.

"[59] Greenwood wrote the title music for Adam Buxton's 2008 sketch show Meebox,[60] and contributed to the 2009 album Basof Mitraglim Le'Hakol by the Israeli rock musician Dudu Tasaa.

[65] That year, Greenwood scored We Need to Talk About Kevin, directed by Lynne Ramsay,[66] using instruments including a wire-strung harp.

[79] Radiohead's ninth album, A Moon Shaped Pool, was released in May 2016,[80] featuring strings and choral vocals arranged by Greenwood and performed by the London Contemporary Orchestra.

[91] For the soundtrack for The Power of the Dog (2021), Greenwood played the cello in the style of a banjo and recorded a piece for player piano controlled with the software Max.

[90] Pitchfork attributed the Smile to Greenwood's frustration with Radiohead's slow working pace and his desire to release records that are "90 percent as good [that] come out twice as often".

[97] The Guardian critic Alexis Petridis said the Smile "sound like a simultaneously more skeletal and knottier version of Radiohead", exploring more progressive rock influences with unusual time signatures, complex riffs and "hard-driving" motorik psychedelia.

[100] On 9 June 2023, Greenwood and the Israeli musician Dudu Tassa released Jarak Qaribak, an album of Middle Eastern love songs.

[104] In May, a drone-based composition by Greenwood for church organ, "X Years of Reverb" — where X is substituted for the age of the building in which it is performed — premiered at the Norfolk and Norwich Festival.

[91][105] On 25 May Greenwood joined protests in Israel calling for the removal of Prime Minister Benjamin Netanyahu, elections for new leadership, and the release of hostages held by Hamas in Gaza.

[16] Guitar.com wrote that Greenwood's playing on Radiohead's debut album, Pablo Honey, was an "exhilarating melange of tremolo-picked soundscapes, chunky octaves, screaming high-register runs and killswitch antics".

[114] In the 1990s, Greenwood developed repetitive stress injury, necessitating a brace on his right arm, which he likened to "taping up your fingers before a boxing match".

[120] For the "My Iron Lung" riff, he uses a DigiTech Whammy pedal to pitch-shift his guitar by one octave, creating a "glitchy, lo-fi" sound.

"[126] Greenwood is a prominent player of the ondes Martenot, an early electronic instrument played by moving a ring along a wire, creating sounds similar to a theremin.

"[128] As production of the ondes Martenot ceased in 1988, Greenwood had a replica created to take on tour with Radiohead in 2001 for fear of damaging his original model.

[74] Greenwood uses a "home-made sound machine" comprising small hammers striking objects including yoghurt cartons, tubs, bells, and tambourines.

"[90] Greenwood's major writing contributions to Radiohead include "Just" (which Yorke described as "a competition by me and Jonny to get as many chords as possible into a song"); "My Iron Lung", co-written with Yorke,[137] from The Bends (1995); "The Tourist" and the "rain down" bridge of "Paranoid Android" from OK Computer (1997);[16] the vocal melody of "Kid A" from Kid A (2000);[33] and the guitar melody of "A Wolf At The Door" from Hail To The Thief (2003).

[2] Messiaen was Greenwood's "first connection" to classical music, and remains an influence; he said: "He was still alive when I was 15, and for whatever reason I felt I could equate him with my other favourite bands—there was no big posthumous reputation to put me off.

[91] In February 2021, Greenwood appeared on the BBC Radio 4 program Saturday Live; his selected "Inheritance Tracks" were "Sweetheart Contract" by Magazine and "Brotherhood of Man" by Oscar Peterson and Clark Terry.