[1] Cassells was born in Dover, Delaware, grew up in the Mojave Desert north of Los Angeles, and began writing poetry in high school.

He graduated in 1979 from Stanford University with a degree in film and broadcasting, and landed a job creating poetry filmstrips in the film division of a publishing house, where he was working when poet Al Young called to tell him that his manuscript had been selected for publication from the 1981 National Poetry Series competition.

Since 1998, he has taught poetry at Texas State University in the MFA creative writing program.

His collection Soul Make A Path Through Shouting was nominated in 1994 for the Pulitzer Prize in Poetry.

Cassell's poems have appeared in numerous anthologies and in such journals as Ploughshares,[5] Indiana Review, AGNI, The Literati Quarterly, Boston Review, Icarus, and Callaloo.