Undlin, 49, joins the 49ers after spending the 2020 season as the defensive coordinator of the Detroit Lions. Prior to his time with Detroit, he spent four seasons (2015-19) with the Philadelphia Eagles as the team's defensive backs coach.
Undlin went to Philadelphia after three years with the Denver Broncos as the team's defensive backs coach (2013-14) and a defensive quality control coach (2012). Prior to Denver, Undlin spent time with the Jacksonville Jaguars as defensive backs coach (2010-11) and a defensive assistant (2009). From 2005-08, Undlin was with the Cleveland Browns beginning as a defensive quality control coach (2005-06) before moving to secondary/assistant special teams coach (2007) and then was promoted to defensive backs coach (2008). He first entered the NFL coaching ranks as a defensive assistant with the New England Patriots in 2004.
Prior to his time in New England, Undlin worked at the collegiate level at Fresno State as the school's defensive backs coach in 2003. He began his coaching career at California Lutheran University as the school's linebackers coach (1998-99) and was later promoted to defensive coordinator (2000-02).
A native of Sauk Centre, MN, Undlin attended California Lutheran University, where he registered 12 career interceptions as a defensive back.