Thomas "Tom" Feargal Hagen burn 1916. He is the informally adopted son of Don Vito Corleone and serves as the family lawyer and consigliere (advisor).
According to backstory established in the various Godfather novels, Thomas Hagen is an orphan, the son of Martin and Bridget Hagen. He also has a younger sister (name unknown). Martin is a carpenter and a violent alcoholic. When Hagen is ten years old, his mother Bridget catches an eye infection that results in blindness, and dies soon after from venereal disease. A few months later, his grief-stricken father drinks himself to death. Hagen and his sister are stuck in an orphanage, but Hagen runs away. The sister ends up in a foster home, but the social agencies of the time do not follow up on Hagen's case. Hagen lives on the streets for more than a year, and he himself develops a similar eye infection.

Hagen first encounters the eleven-year-old Sonny Corleone when Sonny and two older boys wander into a dangerous alley in the Irish part of Hell's Kitchen, an alley in which Hagen is hiding. Sonny and his companions encounter a man selling switchblades and try to buy one. The man pulls a knife and drags Sonny into the alley, while the other boys run away. Hagen recognizes the man as a local pedophile who kills his victims, and defends Sonny by killing the man, using a board with a nail in it. After introducing themselves to each other, Sonny takes Hagen home and persuades his father to take him into the family. Hagen is given warm food and a cot to sleep on, and the Don personally sees to it that Hagen's eye infection is given medical attention. Although the Don never formally adopts him, thinking that this would be disrespectful to Hagen's parents, Hagen thinks of Vito Corleone as his true father.

After graduating from law school, Hagen offers to work for Vito as though he is one of the Don's own sons. His German-Irish ancestry precludes his formal membership in the Mafia, which does not accept non-Italians. But when Vito's consigliere Genco Abbadando is hospitalized with cancer, Hagen acts as the new consigliere to Don Corleone, the first non-Italian to achieve that important position. Hagen's place in the Family becomes permanent after Abbadando's death a year later. Hagen's ascension to consigliere provokes rival Mafia families to refer jokingly to the Corleones, behind their backs, as "the Irish gang". Despite these affronts to Hagen's ancestry, his keen mind and legal knowledge make him a masterful consigliere and integral part of the Corleone Family's empire.

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 approches Woltz, he offers help with some union trouble, as well as getting one of Woltz's actors off of heroin, in return for giving Fontane the part. 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 racehorse, 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. Later on that night, men working for the Corleones steal 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. Unbekownst 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 loved all the Corleones, Hagen always idolized Sonny, and so blames himself when Sonny is murdered by the Tattaglias. 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.

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.

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.