John "Jack" Boyle is a lawyer and ex-husband of Erin Reagan.
Jack Boyle is a defense attorney who was previously married to Erin Reagan. Jack and Erin have one daughter together; Nicky Reagan-Boyle. Jack comes across as cocky and narcissistic and is not well liked by his ex in-laws.
It is mentioned in 1x03 that Jack and Erin married at Morgan Hall and Erin now considers the place bad luck.
Jack is first seen in person in the Season 3 Episode 'Old Wounds' (3x03) when he shows up for Nicky's sixteenth birthday and also opposes Erin in court, he eventually loses the case but tells Erin he may move back to New York to stay, Erin tells him not to make promises to Nicky that he may not keep.
Jack next shows up in the Season 3 Episode 'Framed' (3x12) after Danny is framed for a crime he didn't commit, Jack, at Henry's request agrees to represent Danny but not before warning Henry and Frank to stay out of the way. The situation is resolved and Danny is exonerated.
Jack is not seen again until the Season 4 Episode 'To Protect and Serve' (4x03) Jack tells Erin happily that he has gotten Nicky concert tickets and doesn't understand why Erin is upset until she tells him she had already gotten Nicky tickets and wanted it to be a surprise, Jack gets annoyed and asks Erin for a list of things 'Jack can do right'. Later in the episode it is revealed that Jack is defending a murder suspect in a trial that Erin is prosecuting. The suspect (Delgado) is a federal informant who murdered a father when he discovered that drugs were being smuggled through the ship yard where he worked. Jack attempts to get the witness identification thrown out in a preliminary hearing but the judge rules on the prosecutions side. Delgado gets desperate and grabs one of the guards guns and holds Jack, Erin and the rest of the courtroom hostage. After Erin is shot in the shoulder Jack helps her by applying pressure to her wound and comforting her. The situation is resolved after Danny manages to shoot Delgado when he attempts to remove Erin and use her as leverage to manipulate the police. Jack later goes to Erin's apartment with flowers to see her, they kiss and then go upstairs together.
Season 4 Episode 'Knockout Game' (4x17) Erin breaks off their relationship as Jack wanted to have all the 'fun parts' without the commitment.