Cassilly is a lifelong resident of Harford County, graduating from Bel Air High School in 1984 and enlisting in the 29th light Infantry Division of the Army National Guard. He attended Millersville University in Pennsylvania with assistance from the GI bill.
After college, Cassilly was hired as a certified manual arts therapist at the Perry Point VA Medical Center, working with retired veterans. A year later, Cassilly started his career with Harford County Public Schools, where he has served in many roles over the past 24 years. The first nine years Cassilly was a teacher at both the middle and high school levels. Upon earning his master's degree from Loyola University, Cassilly was a school-based administrator at Fallston High School and later served as hearing officer for long-term suspensions in the Student Service Department at the central office.
As the assistant supervisor of resource conservation for the school system in the facilities department, his responsibility is to reduce operational costs and oversee sustainability initiatives.
Under his direction, the school system has gained statewide recognition for its energy efficiency programs and cost-cutting strategies that allow for school funds to be redirected toward student centered needs. In 2012, Cassilly was honored as "Innovator of the Year" by the Maryland chapter of the U.S. Green Building Council.
Two of Cassilly's brothers have and do hold elected office in Harford County. Joseph Cassilly is the Harford County State's Attorney and Bob Cassilly, a former Bel Air commissioner and mayor and Harford County councilman, is running for Senate in District 34.
Andrew Cassilly's campaign platform for House of Delegates focuses on reducing state taxes by increasing infrastructural and operational efficiencies.
"If we can reduce the waste and dysfunction that currently exists in our state government, we can get a hold on higher taxes" Cassilly said.