Thomas played cornerback with the Chiefs from 1966 to 1978 and remains their all-time leader with 58 interceptions and 938 interception-return yards.
"Coach Thomas is someone that, myself, growing up in the NFL and being around football most of my life, is somebody that I really always looked up to and had a great amount of respect for," Chiefs coach Todd Haley said Monday at a news conference, according to the team's official Web site. "Once I got into the coaching side of the NFL, you heard about coach Thomas. ... When I've seen him in a room or at the combine or at the Senior Bowl or on the opposing sideline, I've always looked at him with a great deal of respect.
"When his name came up, Scott (Pioli, the Chiefs' general manager) has a close relationship with Thomas Dimitroff in Atlanta, and I know everybody there had nothing but terrific things to say about coach (Thomas), and with our situation in the secondary, the more we visited with him –- coach (Romeo) Crennel (the Chiefs' new defensive coordinator) and I actually visited with him at length, along with a number of other guys -– and just agreed that this was the guy for us."
A five-time Pro Bowl pick, Thomas was inducted into the Pro Football Hall of Fame in 2008. He spent the past eight years as an assistant coach with the Atlanta Falcons, with whom he also was assistant head coach/secondary. He was the Falcons' interim head coach for the final three games in 2007. Before that, he was defensive coordinator for the Minnesota Vikings, Green Bay Packers and Philadelphia Eagles.
"I'm so excited to be back, and I'm loving the opportunity," Thomas said. "The main thing I'm here to do is to support coach Ronnie Bradford, who coach Haley thinks is going to be a fine, young secondary coach in the future. I look forward to working with him and helping him out and having him helping me out, and we'll work under the guidance of coach Crennel and we'll see if we can get these guys better and competing."
The Chiefs also announced Monday that they have completed their staff with Bernie Parmalee as tight ends coach and Otis Smith in charge of defensive quality control.
The Associated Press contributed to this report.