His non-Italian ancestry precludes his formal membership in the mafia family, but when the consigliere Genco Abbandando dies, Hagen is given his position.

The novel and first film, set in 1945–1955, portray Hagen aiding Vito and Michael in warring against the other ruling New York Mafia families.

In The Godfather Part II, set in 1958–1959, Hagen serves as Michael's right-hand man during his power struggle with Hyman Roth (Lee Strasberg).

In the novel, Hagen asks to work for Vito after graduating from law school, knowing full well that his adoptive father is the most powerful Mafia chief in the nation.

When singer Johnny Fontane (Al Martino) seeks his godfather Vito's help in securing a movie role that could revitalize his fading career, Vito sends Hagen, accompanied by some of his caporegimes, to Hollywood to persuade Jack Woltz (John Marley), a powerful movie producer, to cast Fontane in his new war film.

Hagen declines, and soon afterward, Woltz awakens in bed with his prized racing stallion's severed head planted under the satin covers, scaring him into casting Fontane in the film.

Hagen goes to the hospital with private detectives who are licensed to carry firearms to protect Vito and stops Captain Mark McCluskey (Sterling Hayden), a corrupt NYPD officer on Sollozzo's payroll, from taking Michael into police custody.

In the novel (and in a deleted scene from the film), Hagen notices that bodyguard Rocco Lampone has been secretly promoted to caporegime and hitman Al Neri reports directly to Michael, rather than through Clemenza and Tessio.

For instance, he is excluded from the negotiations with Hyman Roth (Lee Strasberg) to legitimize the Corleone family by going into business with Cuban dictator Fulgencio Batista.

Assuring their fraternal bond and explaining that he withholds information from him out of admiration, Michael proclaims Hagen acting Don while he leaves and attempts to find out who betrayed him.

The fall of Batista's regime in Cuba forces Michael to temporarily abandon his plans to become a legitimate businessman, and he resumes his role as the Don of the Corleone family.

Near the end of the film, Hagen is unable to disguise his displeasure over Michael's increasing ruthlessness and paranoia, questioning the need to kill an already dying Roth.

Challenged point blank to confirm his loyalty to the Corleone Family, Hagen responds to Michael (in Italian) that he remains loyal.

There is no specific indication in the film as to when or how he died, except that it was prior to his son, Andrew (John Savage), being ordained a Roman Catholic priest.

The role of Corleone family lawyer and advisor is instead held by a new character, B. J. Harrison (George Hamilton), as well as Genco's grandson Dominic Abbandando (Don Novello).

Hagen was originally intended to have been featured in The Godfather Part III, but was written out due to a salary dispute between Duvall and the film's producers.

[2] Coppola has stated that Part III was to feature a split between Michael and Hagen as its central plot, as seeds of dissension were planted in the first two films.

[3][4] The Godfather Returns, Mark Winegardner's 2004 sequel to Puzo's original novel, portrays Hagen's role as consigliere in the first few years after Michael ascends to the head of the family.

The novel, which covers the period from 1955 to 1962, portrays Hagen once again acting as Michael's right-hand adviser and taking an important role in the Corleones' dealings with a powerful political family, the Sheas (analogous to the Kennedys).

The novel also portrays Hagen covering up for Michael's brother Fredo when he kills a man in San Francisco, and bailing him out of jail when he attacks his wife's lover.

When Michael has Fredo killed (as originally portrayed in The Godfather Part II), Hagen suspects what really happened, but remains willfully ignorant.