Thomas agreed to a one-year, $2 million deal, a source told ESPN.com's John Clayton.
Thomas, 36, started 14 of 16 games for the Dallas Cowboys in 2008 during his 13th NFL season, recording 94 tackles, 65 of which came solo.
His addition raises some question what the Chiefs might do in the draft. Many are thinking the Chiefs could draft linebacker Aaron Curry with the third pick. Curry is versatile enough to play outside or middle linebacker.
So far, though, general manager Scott Pioli has three potential starting linebackers -- Thomas, Johnson and Vrabel. He's also brought in two backups -- Monty Beisel and Corey Mays. On the roster, the Chiefs also have another potential starter in Demorrio Williams.
The Chiefs also waived wide receiver Will Franklin on Monday. Franklin made one start last year and appeared in 13 games.
Thomas spent the first 12 years of his career with the Miami Dolphins, with whom he was named to his last Pro Bowl in 2006 before missing 11 games with migraine headaches resulting from an auto accident in 2007.
Thomas made off-the-field news in March when he told The Dallas Morning News that his wife, Maritza, was handcuffed and spent about three hours in jail in July 2008 after Dallas police officer Robert Powell pulled her over for an illegal U-turn.
Powell, who has resigned, stopped Houston Texans running back Ryan Moats last month for running a red light on his way to the hospital to see his dying mother-in-law. Powell detained Moats for more than 10 minutes, in which time his mother-in-law died.
"We wanted to tell our story, not knowing how many others have been affected by Officer Powell," Thomas told the newspaper. "We know the vast majority of the Dallas police force are good and professional people, but this guy just seems excessive."
Information from ESPN.com's John Clayton was used in this report. Information from The Associated Press was used in this report.