While at Rider College in 1972-1973, John was hired by the Office of Public Defender in Trenton as a part time assistant investigator. Upon graduating from Vermont Law School, he was accepted as Law Clerk to the Honorable Herman Belopolsky, J.C.S., Honorable Harold B. Wells, III, A.J.S.C. and Honorable W. Thomas McGann, J.C.S.

Upon passing the Bar in New Jersey in 1976, he joined the firm of Mathews and Sitzler as an associate and was made partner the following year.

In 1979, John became Solicitor for the Wrightstown Borough Planning Board as well as Municipal Prosecutor for Pemberton Township. Over the next few years John became a Solicitor for the Delran Township Planning Board as well as Municipal Prosecutor for many municipalities including Hainesport, Cinnaminson, Pemberton, Mount Holly, New Hanover, and Springfield.

In 1990, he and his father, John O. Sitzler, Jr., formed the partnership of Sitzler and Sitzler and in 1995 became the Sole Proprietor of Sitzler and Sitzler when his father became of counsel to the firm.

From 2000-2013 John taught as an adjunct Professor of Law at Widener University School of Law. Wilmington, Delaware campus and taught Alcohol, Vehicle, and the Law, one of the most popular courses at Widener.

From 2007-2014, John was nominated by his peers as the Top Attorney to work with on DWI defense cases for SJ Magazine. He was also voted as Top DUI attorney by the public for South Jersey Magazine for the years 2010-2014.