He played Tier II Junior "A" for the Kanata Valley Lasers in 1988-89 and then joined Boston U. With Boston he won the Hockey East rookie-of-the-year in 1989-90 and led his team to the 1990 Frozen Four. In the 1991 Frozen Four his team lost the final 8-7 in overtime to the Northern Michigan Wildcats. Cashman also won the Eberly Trophy in 1990, 1992 and 1993.
In 1993-94 Cashman turned pro with the Detroit Falcons and then went to the Adirondack Red Wings. While playing for Adirondack he cracked his tailbone. He returned to Ottawa and was diagnosed with Ankylosing Spondylitis, an inflammation of the vertebrae, which could lead to arthritis of the spine. Doctors told him if he continued to play, he might not be able to walk at 30. He immediately retired from pro hockey.
He died in 2008 of heart problems. Cashman was just 39.