His league and championship career with the senior Meath county team spanned sixteen seasons from 1974 to 1990.
Cassells made his debut on the inter-county scene at the age of seventeen when he was selected for the Meath minor team.
He enjoyed one championship season with the minor team, ending the year as a Leinster medal winner.
Over the course of the next sixteen years, Cassells won back-to-back All-Ireland medals, beginning in 1987 before collecting a second as captain of the team in 1988.
As a regular member of the Leinster inter-provincial team on a number of occasions, Cassells won back-to-back Railway Cup medals in 1985 and 1986.