Ashley was born in Schefferville, Quebec, Canada, in 1961. He emigrated to England in 1977, where he joined the regimental band of the Royal Scots. He served in the army for 6 1/2 years. After leaving, he worked in England and Europe as a session trombonist, working with, among others, Carla Bley, George Russell and Andy Sheppard. In the late 80s Slater released his first album on Antilles (Island) with the band Microgroove.