The 22-year-old actor is appearing in the planted pilot - which will air as part of 'Criminal Minds' current season - as tech analyst Monty, reported Deadline.
Monty is described as "sweet and incredibly smart", and stays at home while the rest of the team are abroad.
Williams is currently recurring on 'The Walking Dead' as Noah, and starred in Matthew Perry comedy Go On and The CW's Everybody Hates Chris.
Gary Sinise was previously announced to be playing the lead of the 'Criminal Minds' off-shoot as team leader Jack Garrett.
Erica Messer, Mark Gordon and Nick Pepper are executive producing the project.