Horton has coached in the NFL for 17 seasons and has been with the Steelers for seven years, the past four as defensive backs coach.
Horton, who played in the NFL for 10 seasons, becomes the Cardinals' third defensive coordinator in Whisenhunt's four years in Arizona. He replaces Bill Davis, who was fired after two seasons on the job.
Horton, 50, worked for 11 years with Steelers defensive coordinator Dick LeBeau, at Cincinnati and at Pittsburgh.
Defense has been a persistent trouble spot for much of Whisenhunt's time in Arizona. The defense showed some improvement during the 2009 season, then surrendered 90 points in two playoff games.
In going 6-10 this past season, Arizona ranked 29th in total defense, 23rd in passing defense and 30th in rushing defense.
Horton is the fourth coach on Arizona's staff who worked with Whisenhunt in Pittsburgh.
Russ Grimm, who carries the complicated title of assistant head coach, running game coordinator and offensive line coach, came to the Cardinals as Whisenhunt's right-hand man after being a finalist for the Arizona head coaching job.
Special teams coach Kevin Spencer and linebackers coach Matt Raich also came with Whisenhunt from Pittsburgh.
Copyright 2011 by The Associated Press