[2][3][4][5][6][7] Grennan's career spanned several of the richest and most diverse decades in popular music, and he worked with a large number of the most famous artists of his time, both in Jamaica and in the United States.

He played on sessions for numerous Jamaican acts notably Toots and The Maytals, Bob Marley, the Wailers, Jimmy Cliff and Desmond Dekker.

He mentored a number of Jamaican musicians including notable drummers, Carlton Barrett, Sly Dunbar, Tin Leg, and Willie Williams.

His work with Kenyatta (including the acclaimed "Stompin at the Savoy" album, and several others) combined with his unique, innovative style, provoked interest from diverse musicians.

Over the years, Grennan continued to back Toots & the Maytals, and a few other touring Jamaican acts including Pat Kelly, the Clarendonians, Yellowman and the Skatalites.

In the last year of his life Grennan released his final record, Clean Slate, as well as performing on and sharing arrangement duties with dub-poet Anthony Pierre on his debut, Obeah Accompong.

[citation needed] In early 2000, Grennan appeared on Back to the Island, a reggae compilation album of local tracks produced by Peter Simon, of Martha's Vineyard.