Rex is a native Floridian having been born and raised in the small town of Shalimar in the Florida panhandle. After graduating from Tulane University in 1975 with a degree in political science, Rex returned to Florida where he graduated from the Florida State University College of Law in December 1977. Rex moved to Lakeland in February 1978 and began his legal career as an assistant public defender. Between 1980 and 1984, Rex served as a prosecutor under State Attorney Quillian Yancey.

After managing his own law office for four years, Rex returned to the Office of the Public Defender in July 1988. During his time as an assistant public defender, Rex supervised every trial division within the office, including misdemeanors, felony, juvenile, and capital murder. Before his election as Public Defender in 2012, Rex served as the Administrative Division Director for 10 years. Rex served as the Vice President of the Florida Public Defender Association from July 2017 to July 2019.