Alexander Harper Berkeley is an American actor. Since beginning his career in the early 1980s, he has appeared in over 200 film and television projects. His film roles include Terminator 2: Judgment Day (1991), Candyman (1992), Barb Wire (1996), Air Force One (1997), Gattaca (1997), and Shanghai Noon (2000). He also appeared in the crime drama Heat (1995) after being cast in L.A. Takedown (1989), an earlier rendition of the film's script. On television, he headlined the Citytv psychological thriller The Booth at the End (2010–2012) and was a series regular on the Fox action drama 24 (2001–03) and The CW action thriller Nikita (2010–2012). As a guest star, Berkeley portrayed Sheriff Thomas McAllister on the CBS drama The Mentalist (2008–13) and Gregory on the AMC post-apocalyptic horror The Walking Dead (2016–18).