[2] Cassilly graduated from Bel Air High School in 1976, and later attended Johns Hopkins University, where he was a member of the Reserve Officers' Training Corps and earned a B.A.

[4] In February 2006, Cassilly was activated by the United States Army and deployed in Iraq, where he served as a senior governance advisor in the Karbala Governorate and as a strategic planner for the U.S. Embassy in Baghdad until 2010.

Following this decision, Cassilly changed the locks on Bennett's office and restricted access to his government email account.

In a statement to Maryland Matters, Cassilly acknowledged monitoring Penman's emails, but denied any wrongdoing, asserting that county policy allowed for such reviews and attributing his accusations to politics.

The restrictions appear to have been placed in response to budget disputes from earlier in his term, including Cassilly's opposition to a new facility for the Harford County Sheriff's Department.

[28] In late September 2023, Healey threatened to sue Cassilly after he blocked her access to the email account of a "key employee who is on emergency family leave".

[37] In March 2024, the Maryland Public Information Act Compliance Board found Cassilly's administration in violation of the Public Information Act after refusing to fulfill a request by Harford County Sheriff Jeffrey Gahler to disclose emails connected to a feasibility study conducted on the county's behalf by architectural firm Manns Woodward Studios.

[38] During the 2017 legislative session, Cassilly voted against a bill that would give the Attorney General of Maryland the ability to sue the federal government.

[39] He also opposed a bill that would give investigators in the Baltimore State's Attorney's Office police powers.

[44] In 2003, Cassilly said he opposed a proposal to limit the height of buildings in downtown Bel Air to five stories, expressing concerns that it would have unintended effects on the area.

[55] Cassilly opposes the Blueprint for Maryland's Future, an education reform package passed by the legislature in 2021.

[64] In October 2024, after the county school system and sheriff's office requested $80 million in additional funding for wage increases, Cassilly posted a video to Facebook telling school officials to "stop playing politics" and to "face to the fiscal reality", citing two consecutive years of low revenue growth in the county.

He added that the county would need to raise property taxes by 20 percent to afford the wage increases, which he refused to do.

[71] In November 2023, Cassilly opposed a proposal that he claimed would house about 220 undocumented immigrants in Harford County-area hotels, saying that he would "use every resource at my disposal to prevent it".

State officials told The Baltimore Sun shortly after Cassilly made this statement that there was no such plan in the works.

[76] During the 2019 legislative session, Cassilly introduced a bill to prohibit smoking or consuming marijuana in a vehicle on the highway.

[77] In 2019, Cassilly voted against a bill that would raise the state's minimum wage to $15 an hour by 2025, calling it a "direct assault" on companies that offer entry-level positions.

[78] During the 2015 legislative session, Cassilly introduced a bill that would allow evidence of previous sexual assault allegations to be brought in to criminal trials.

[79] In March 2020, he wrote to Republican members of the Maryland House of Delegates urging them to vote against a bill to extend the statute of limitations on child sexual abuse lawsuits, comparing the bill to the sexual assault accusations made against U.S. Supreme Court Justice Brett Kavanaugh during his nomination.

[83] In January 2018, Cassilly objected to remarks made on the floor of the Maryland Senate condemning comments made by President Donald Trump calling Haiti, El Salvador, and African nations "shithole countries", saying that the remarks were "not appropriate matter for the Senate of Maryland".

[86] During the 2020 legislative session, Cassilly voted to sustain Governor Larry Hogan's veto of a bill that would limit employers from asking a job applicant's criminal record.