The song was rumoured to be about smoking dried banana skins, which was believed to be a hallucinogenic drug in the 1960s, though this aspect of bananas has since been debunked. According to Donovan's notes, accompanying the album Donovan's Greatest Hits, the rumour that one could get high from smoking dried banana skins was started by Country Joe McDonald in 1966, and Donovan heard the rumour three weeks before "Mellow Yellow" was released as a single. According to The Rolling Stone Illustrated Encyclopedia of Rock and Roll, he admitted later the song made reference to a vibrator; an "electrical banana" as mentioned in the lyrics. This definition was re-affirmed in an interview with NME magazine: "it's about being cool, laid-back, and also the electrical bananas that were appearing on the scene - which were ladies' vibrators."
Paul McCartney can be heard as one of the background revellers on this track, but the "quite rightly" whispering answering lines in the chorus is not McCartney but rather Donovan himself. Donovan had a small part in coming up with the lyrics for "Yellow Submarine", and McCartney played bass guitar (uncredited) on portions of Donovan's Mellow Yellow album.
"Mellow Yellow" was covered in 1967 by soul singer Big Maybelle on her album Got a Brand New Bag. It was also covered in 1968 by British R&B singer/keyboardist Georgie Fame on his album The Third Face of Fame.
^Laing, Dave (1975). The Electric Muse: the story of Folk into Rock. Methuen. p. 151. ISBN0-413-31860-5. Donovan (...) did change styles to make a couple of enormously successful pop singles, "Mellow Yellow" and "Sunshine Superman", before disappearing from the front ranks.
^Buckley, Peter (2003). The Rough Guide to Rock. Rough Guides. p. 305. ISBN978-1-8435-3105-0. A further move into psychedelic pop spawned another million-seller in "Mellow Yellow" — [Donovan's] best-known song
^Ellis, Iain (2012). Brit Wits: A History of British Rock Humor. Intellect Books. p. 42. ISBN978-1-8415-0565-7. Hits like "Mellow Yellow" (1967) and "Hurdy Gurdy Man" (1968) saw Donovan become the public face of fanciful British psychedelic pop in the latter part of the decade.