Theo discovered that Robert Quarles's father was a heroin addict who forced him to have sex with other men. Horrified, Theo helped Quarles to kill his father and adopted him, much to the displeasure of his son Sammy Tonin. In 1983, he killed a government informant with his own hands, an act that was witnessed by Drew Thompson. When Theo discovered that Drew had witnessed the event, he attempted to kill him, but Drew shot him in the eye and left him for dead, escaping with $2 million worth of cocaine.
In the episode "Measures", Theo, angry that Quarles threatened to kill Sammy after he gave him his inheritance instead of Quarles, sends two assassins, Partlow & Sarno, to kill Quarles, however the two are arrested by the US marshals. Theo receives a call from Dixie Mafia associate Wynn Duffy, who asks him if he intends to try and have Quarles killed again. When Theo replies that he does, Duffy tells him that if he agrees to make him the head of the oxycontin business once Quarles is dead, he will help him to kill Quarles. Theo is initially reluctant, but agrees.
He also briefly appears in "Slaughterhouse", in which he receives a call from Quarles asking to be allowed to return to Detroit. Theo agrees, but only if Quarles pays him $500,000 in restitution for his earlier actions. Quarles attempts to get this money from local crime boss Ellstin Limehouse, but Limehouse ultimately betrays and murders him.
In the third episode of the fourth season, "Truth And Consequences", Theo is the unseen main antagonist, as his henchman Mason Goines abducts Drew Thompson's former wife Eve Munroe in order to find out where he is. Munroe tells Goines that Drew is near Louisville, but corrupt FBI agent Alex Barnes confirms this isn't true and Theo orders Goines to execute Munroe. Fortunately, the marshals manage to find Munroe and Goines is arrested, foiling Theo's plan.
It is later revealed in the season four finale "Ghosts" that Theo has gone to Tunisia, leaving Sammy, the new head of the Detroit branch of the Dixie Mafia, in charge of his affairs until he returns.
In the episode "Shot All To Hell", Theo discovers that Dixie Mafia member Ethan Picker murdered Sammy on the direction of mobsters Al Sura and Scott Fenton, and orders Elias Marcos to kill all those responsible. After killing Sura, Marcos goes after Picker, but is ultimately killed by US marshal Raylan Givens while attempting to kill him. While searching the warehouse where Marcos was killed, the marshals find bloodstains, and discover they lead to a lawn chair containing the bleeding and bandaged Theo, revealing that he was in fact recovering from a bullet wound and not in Tunisia as previously thought. Marshal Art Mullen mockingly says "Hello, Theo" as the marshals arrest him.
He is also brought up in the episode "The Toll", in which Art is shot by an unknown assailant, injuring him. Several marshals hypothesise that Theo attempted to have him killed as revenge for arresting him. Marshal David Vasquez accuses Picker of shooting Art on Theo's orders, but this theory is shot down when Picker points out that Theo tried to have him killed, so he wouldn't work for him. It is ultimately revealed that the attacker was in fact Daryl Crowe Jr., and the imprisoned Theo had nothing to do with it.