"C'mon now! Has it really been 30 years! 'Dirty Steve' cleans up pretty damn good, don't he! So overjoyed to be reunited with my old saddle mate, Dermot Mulroney! Oh yeah, the bullets are gonna fly! #YoungGuns #ProdigalSon @JohnFusco12 #Pals @prodigalsonfox @ProdigalWriters," Phillips tweeted Tuesday.
Phillips also shared a photo of him, Mulroney and Prodigal Son co-star Bellamy Young.
Deadline.com said Mulroney will play a wealthy murder witness in the last five episodes of the show's first season. His first episode is to air on March 30.
Prodigal Son casts Tom Payne as Malcolm Bright, a consultant for the New York Police Department and the son of Dr. Martin Whitly, a serial killer played by Michael Sheen.
Phillips plays NYPD Lt. Gil Arroyo, who helped capture Whitly and has been looking out for his family since.
Mulroney has since starred in dozens of films and TV projects, including My Best Friend's Wedding , The Family Stone, Enlightened, August: Osage County and Shameless. He can now be seen in Four Weddings and a Funeral and The Righteous Gemstones.
Phillips has also been a fixture on the big and small screens for several decades, memorably co-starring in Longmire for six seasons.