Cornick was born on April 23, 1947 in Barrow-In-Furness and played in various groups before briefly joining Ian Anderson in the John Evan Band in 1967. The band soon split, leading the pair, along with guitarist Mick Abrahams, to form what would become Jethro Tull.

The bassist played on the first three Tull albums This Was, Stand Up and Benefit before he was "invited to leave" by the band's manager Terry Ellis in 1970.

He formed Wild Turkey, recording the albums Battle Hymn, Turkey and four-track sampler Don't Dare To Forget, before joining German band Karthago. He went on to form Paris with ex-Fleetwood Mac member Bob Welch.