Joining the Family
Tom Hagen was orphaned as a child and spent an entire winter on the streets of New York. He then met Sonny, who brought him home with him. The Corleone family took Tom in and Vito had a doctor cure him of an eye infection he had picked up from his late mother. Vito urged Tom not to take on the family name as a show of respect to his late parents; nevertheless Tom became fully integrated into the family.
Though his late childhood on the streets forced him to develop life-preserving survival skills, the adult Tom rarely if ever shows passion outwardly. He is happily married to a woman he respects and even admires (see The Godfathers Revenge), but also has a mistress. We learn repeatedly that he has little or no interest in entertainment or entertainers, even though the Corleone Family does a great deal of business in Hollywood. The novels reveal that he paid close attention to Vito Corleone while growing up and learned well his skills at analyzing the subtleties of human behavior (as well as the Sicilian dialect). Tom sees as one of his greatest assets his ability to blend in in the "outside world" (see The Godfather Returns), since his manner is so lacking in any kind of ostentation. He is thus able to accomplish things in the public sphere such as scanning a small town's records to find birth certificates his associates could use for false identities and leave almost no trace in the memories of those around him.
After Connie Corleone's wedding, Hagen is dispatched by Vito Corleone to Hollywood in order to convince Jack Woltz, a big-time movie studio head, to give singer/actor Johnny Fontane (Vito's godson) the lead role in his new war film. When he first approaches Woltz, he offers help with some union trouble. Hagen then tells Waltz that one of his actors has moved from marijuana to heroin. A deleted scene in the movie shows that this information was going to be used to expose the star and thus hurt Woltz's studio. Woltz at first angrily refuses, but becomes more cordial once he finds out who Hagen works for. Woltz invites him over to his palatial estate for dinner, and shows him his prized stud horse, Khartoum. During the dinner, Woltz tries to work out another deal with Hagen, but refuses to cast Fontane, who had slept with one of his mistresses. Men working for the Corleones break into Woltz's stables and decapitate Khartoum, and place the horse's severed head and a large amount of its blood in Woltz's bed. The next day Hagen receives a call from a ranting Woltz, who threatens to bring the law down on the Corleones' heads. Hagen gives a nonchalant response and hangs up. Shortly afterwards, Woltz gives Fontane the coveted role.
Hagen later compiles information on drug lord Virgil "the Turk" Sollozzo, who had approached Vito Corleone on behalf of the Tattaglia crime family to help fund and provide political protection for a heroin operation in New York, in exchange for 30 percent of the profits. Vito, after considering his options, refuses the Turk's proposal, though Sonny shows a slight interest.
That December, Hagen is abducted by the Turk and his bodyguards. At an undisclosed location, Sollozzo informs Hagen that Don Corleone has been shot and killed, and tells Hagen to convince Sonny to go along with the original deal. Hagen promises to calm Sonny down, but warns the Turk about inevitable reprisal from Luca Brasi, the Don's fanatically loyal bodyguard and hitman. Unbeknownst to Hagen, Brasi was killed by Sollozzo and Bruno Tattaglia. The meeting is interrupted when Sollozzo receives word that Don Corleone survived the shooting, which ruins all of Sollozo's plans, as Sonny would listen to no deal while his father was still alive.
While he loves all the Corleones, Hagen idolizes Sonny, and so blames himself when Sonny is murdered by the Barzinis. After becoming the new head of the family, Michael Corleone removes Hagen as consigliere, restricting him to handling the Family's legal business in Nevada, Chicago, and Los Angeles. Hagen accepts the decision, and remains loyal. However, after Vito's death, Hagen resumes his role as consigliere.
The Godfather: Part II
After an attempt on Michael's life in The Godfather, Part II, Hagen takes over as acting Don while Michael tries to find out who in his organization had betrayed him and aided the assassins. Hagen is instrumental in both securing the friendship of powerful Senator Pat Geary and defending Michael during the Senate hearings on the Mafia.
The fall of Fulgencio Batista's regime in Cuba forces Michael to abandon his dream of becoming a legitimate businessman and retake his place as the Don of the Corleone family. As a result, he gives Hagen back his old position as consigliere.
Even as Michael becomes increasingly ruthless and paranoid, Hagen dutifully fulfills his role as not just a legal adviser but a dispassionate envoy for the Family. For example, he gives Frank Pentangeli, who had betrayed Michael, the "option" of committing suicide so that Pentangeli's family would continue to be taken care of after his death.
The Godfather: Part III
According to The Godfather: Part III, Hagen dies at some point prior to the timeframe of the film, 1979-1980. There is no specific indication in the film as to when or how he dies, except that it is before the ordination of his son, Andrew, a Roman Catholic priest.
The Godfather's Revenge
Mark Winegardner's sequel The Godfather's Revenge explains that Hagen is murdered in August 1964 by former Corleone underboss Nick Geraci, who drowns him in the Florida Everglades. Geraci then sends Michael a package containing a dead baby alligator along with Hagen's wallet, a message similar to the one that is sent to Sonny in the original novel following Luca Brasi's death.
Behind the scenes
The Hagen character was originally intended to have been featured in The Godfather, Part III, but was written out because of a financial disagreement between Duvall and the film's producers. Coppola has stated that Part III was originally planned to feature a split between Michael and Hagen as its central plot. Coppola stated in the film's commentary that Duvall demanded the same salary as Al Pacino (who portrayed Michael Corleone).
Vito Corleone (Marlon Brando) - Unofficial adopted father Carmella Corleone - Unofficial adopted mother Santino 'Sonny' Corleone (James Caan) - Unofficial adopted brother Theresa Hagen - Wife Frank and Andrew Hagen (Roman Coppola and Gian-Carlo Coppola) - Sons