Williams had the same job with the Los Angeles Rams for the past three seasons.
Williams, best known for his role in a bounty for injury scandal that brought a suspension in 2012, had the same job with the Los Angeles Rams for the past three seasons.
His unit ranked ninth in the 32-team league in total defense this past season.
"We are thrilled that Gregg Williams is joining our staff," said Browns coach Hue Jackson. "Gregg has been an outstanding defensive coach in this league for a very long time. He adapts his scheme to maximize the strengths of his personnel and has always been able to get the most out of his players. We look forward to him coming in, helping our players improve and helping us become (a) winning team."
Under Horton's guidance, the Browns were next-to-last in total defense and run defense.
"I have so much respect for Hue having competed against him in this league for so many years," Williams said. "We're looking forward to getting to work."
Williams, 58, had served as defensive coordinator for the New Orleans Saints when he was suspended by the NFL in 2012 for his role in the Saints' Bountygate scandal, offering rewards for injury-inflicting hits.
Horton, 56, had been fired as an assistant coach by the Browns in 2013 after a 4-12 campaign that also saw head coach Rob Chudzinski axed after his first season. Horton had been at Tennessee before leaving to join Jackson's staff when he took over the Browns last year.
Williams went 17-31 in three seasons as head coach of the Buffalo Bills from 2001-2003. The Browns become the sixth club he has served as defensive coordinator after Tennessee, Washington, Jacksonville, the Saints and Rams.
The Rams fired head coach Jeff Fisher last month and have yet to hire a replacement, leaving assistant coaches free to seek other NFL postings.