He was a founding member and the bassist of the 1960s English rock band the Yardbirds, which launched leading guitarists Eric Clapton, Jeff Beck and Jimmy Page to fame.

"[6] While in the Yardbirds, he co-produced and engineered much of their music, working with record producers such as Mickie Most, Simon Napier-Bell and Giorgio Gomelsky.

He went on to become a successful producer with credits including Cat Stevens' albums Tea for the Tillerman (1970), Teaser and the Firecat (1971) and Catch Bull at Four (1972).

An article in The Washington Post praised Samwell-Smith's "deft, understated touch" on these recordings as a primary reason for their commercial success, and commented: "The chamber ensemble palette Samwell-Smith employed, consisting mainly of acoustic guitars, piano, upright bass and hand percussion, and the refined arrangements he crafted, perfectly complement the interior landscapes that Stevens was exploring.

"[7][5] Samwell-Smith also produced recordings for Jethro Tull,[5] Carly Simon,[5][8] Renaissance, Murray Head, Chris de Burgh, Beverley Craven, Toto Coelo, Illusion and Claire Hamill.