Matt Patricia continued re-stocking his coaching staff.
It's the Lions' latest hire after changing a bevy of assistants this offseason, including former defensive coordinator Paul Pasqualoni, who stepped away.
Undlin's first NFL coaching experience came in 2004 with the New England Patriots as a defensive assistant, also Patricia's first season with the Pats.
The 48-year-old Undlin spent just one season in New England before moving to Cleveland, then Jacksonville, then Denver, and finally landing in Philadelphia the past five seasons as its defensive backs coach.
A coach highly thought of in Philly, Undlin will get a chance to run a defense, though Patricia is likely to call plays in a pivotal season for him in Detroit. Undlin was believed to be a candidate to take over had Eagles DC Jim Schwartz earned the Browns' head coaching gig. With Schwartz returning to Philly, Undlin leaps to his first DC role.
Eagles corners struggled mightily in 2019, but the perception in some circles is that personnel and injuries were a bigger problem than coaching.
Undlin will help take over a Lions defense that couldn't stop a one-legged zebra from grazing in an open field in 2019. If he doesn't help turn that unit around in 2020, Patricia is likely out of a job, and the new DC will quickly become a former DC.