He was also a law clerk for Antonin Scalia, then a judge on the United States Court of Appeals for the District of Columbia Circuit, from 1984 to 1985.

Cassell later wrote a letter to President Obama[7] requesting a sentence commutation, and thereafter prosecutors agreed to release Angelos.

In 2005, Chief Justice Rehnquist appointed Cassell to serve as the Chair of the federal Judicial Conference's Committee on Criminal Law.

In 1996, Cassell represented victims and surviving family members of the Oklahoma City bombing[9] in efforts to obtain access to court proceedings.

In 2018, Cassell filed a petition on behalf of four "Jane Does"[14] urging the Utah Supreme Court to appoint special prosecutors to pursue four rape cases.

Cassell has testified before congressional committees[15] several times, supporting an amendment to the United States Constitution protecting crime victims' rights.

Cassell and Edwards filed a petition for rehearing en banc, which the Eleventh Circuit granted[29] in August 2020.

Cassell has led a legal challenge to the deferred prosecution agreement between the Justice Department and Boeing concerning the crashes of two 737 MAX aircraft.

[35] On January 5, 2024, a doorplug blew off an Alaskan Airlines 737 aircraft, revealing problems with Boeing's production processes.

[36] Then, on July 7, 2024, the Department of Justice notified the judge handling the case that it had reached a plea agreement with Boeing to resolve the matter.

[42][43] In 2020, Cassell argued that the "Minneapolis Effect" was responsible for the homicide and shooting spike that occurred beginning in the summer of 2020.

Cassell has also questioned the frequency with which innocent persons have been wrongfully convicted for crimes they did not commit, calling for close scrutiny of cases involving especially vulnerable defendants rather than a general approach to the issue.

A majority of the Court ultimately rejected Cassell's argument, with Justices Scalia and Thomas dissenting to endorse the position.

Cassell has published a detailed empirical analysis[48] concerning the harmful effects of the Miranda decision, including a lengthy 2017 co-authored law review article containing regression analysis linking declining crime clearance rates after the Miranda decision to the restrictions imposed on police.