[1][2][3] During law school, Melloy interned at the Jo Daviess County, Illinois attorney's office.

Melloy practiced primarily civil litigation and also administrative law, real estate, bankruptcy, and tax as an associate and then as a partner at O’Connor & Thomas in Dubuque from 1974 until 1986.

[2][3] He also regularly co-chaired Tom Tauke's congressional campaigns and served in various capacities for Catholic organizations, including the Dubuque Catholic Metropolitan School Board, Clarke University, and the Knights of Columbus.

[3] Melloy a judge of the United States Bankruptcy Court for the Northern District of Iowa from 1986 to 1992.

[1] On April 2, 2018, the Supreme Court of the United States appointed Melloy as Special Master in the case of Texas v. New Mexico and Colorado.