Mahon received his bachelor's degree from the Florida State University in 1978.

In addition, he received his Juris Doctor from Florida State University in 1981.

After becoming Interim Chief Judge of Florida's 4th Circuit Court based in Jacksonville, on July 1, 2015, Mahon issued a Summary Order prohibiting photography in public places, criticism of the court and specifically "speech that ‘degrade[s] or call[s] into question the integrity’ of judges on courthouse sidewalks" of the Duval County Courthouse,[4] a move that was denounced by lawyers and activists as unconstitutional due to violation of the First Amendment.

On the same day, PINAC and local activists filed federal lawsuit against Mahon's order, alleging prior restraint of free speech.

[6] On July 15, Mahon rescinded the order completely after increasing media attention and criticism.