John Maciejczyk graduated from the United States Naval Academy in 1976. He was stationed on destroyers in the Atlantic Fleet from 1976 to 1981. He was awarded the Navy Achievements Medal and other citations. He taught anti-submarine warfare at the Atlantic Fleet Training Center in 1980-81.
Maciejczyk graduated from Notre Dame Law School in 1984. He clerked for Chief Judge Ruggero Aldisert on the Third Circuit from 1984 to 1986. He worked in private practice in Houston, Texas, and Elkhart, Ind., from 1986 to 1998. He then became the chief deputy prosecutor for St. Joseph County from 1999 to 2002, and Elkhart County deputy prosecutor thereafter until hired as an assistant United States attorney in 2004. His current practice is 50 percent appellate work, 50 percent cybercrime and human trafficking.