Undlin attended California Lutheran University, where he played football as a safety from 1990 to 1994. He had the distinction of being named Second Team All-Southern California Intercollegiate Athletic Conference honors in 1993.
In 2004, Undlin joined Belichick as a defensive coaching assistant with the New England Patriots. When the Patriots won Super Bowl XXXIX that year and then-Patriots defensive coordinator Romeo Crennel left to become head coach of the Browns, Undlin followed Crennel to the Cleveland Browns as a defensive quality control coach. In 2007, Undlin was promoted to secondary coach and assistant special teams coach. In 2008, Undlin was promoted to defensive backs coach. In 2009, Undlin moved to the Jacksonville Jaguars as a defensive assistant following Crennel's firing in Cleveland. He was promoted to defensive backs coach in 2010, but was fired along with the rest of the staff after the 2011 season. In 2012, he was hired as a defensive quality control coach for the Denver Broncos, and he was promoted to defensive backs coach in 2013. With the Broncos firing head coach John Fox, Undlin was hired by the Philadelphia Eagles as defensive backs coach in January 2015.