Cassilly is a Republican who represented district 35B in the Maryland House of Delegates from 2015 to 2019 when he resigned to work for Hogan.

[5] The bill passed the House of Delegates and the state Senate by votes of 137-0 and 45-0, respectively, and was signed by Governor Hogan in April.

[9] In February 2019, following lobbying from Maryland bicyclists, Cassilly worked with delegate Jay Jalisi to introduce House Bill 398, a "safe passing" bill that would allow motorists to cross a double-yellow line in order to safely pass a bicycle.

[11] In December 2020, Governor Hogan appointed Cassilly to serve as his senior advisor, replacing Keiffer Mitchell Jr., a Democrat who he had recently appointed to the role of Chief Legislative Officer.

[12] Hogan would appoint Mike Griffith to replace him in the House of Delegates on December 30, 2019.