Undlin began his coaching career at his alma mater California Lutheran as a linebackers coach from 1998 to 1999. He was promoted to defensive coordinator in 2000, a position he held through 2001, when he moved to Fresno State University. He spent two seasons in 2002 and 2003 as a graduate assistant at Fresno State under head coach Pat Hill, a former assistant coach with the Cleveland Browns under Bill Belichick.
He had the distinction of being named second-team All-Southern California Intercollegiate Athletic Conference honors in 1993. College Professional In 2004, Undlin joined Belichick as a defensive coaching assistant with the New England Patriots. 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.
He was retained as defensive backs coach on January 20, 2016.
