The film uses a nonlinear narrative, alternating between J. Edgar Hoover's role in establishing the Federal Bureau of Investigation (FBI) and his later years trying to safeguard it against perceived threats.

In 1919, Attorney General A. Mitchell Palmer survives an assassination attempt by anarchists and assigns Hoover, a Justice Department employee, to head a division dedicated to purging radicals.

Realizing that the police handling of the crime scene was primitive, and recognizing the importance of criminal science, he informs his mother and moral guide Annie of recent events.

Hoover then meets newly-hired secretary Helen Gandy and takes her to the Library of Congress, where he demonstrates the card catalog system he created, claiming that solving crimes would be easier if every citizen were easily identifiable.

Despite his close monitoring of suspected foreign radicals, Hoover finds that the Department of Labor is unable to deport anyone without clear evidence of a crime; however, Anthony Caminetti, commissioner general of immigration, dislikes prominent anarchist Emma Goldman.

Though Charles Lindbergh Jr. is found dead, these techniques lead to Bruno Richard Hauptmann being arrested, convicted, and sentenced to death for the crime; the case elevates the FBI to prominence.

Following an embarrassing line of questioning by Senate Appropriations Committee chair Kenneth McKellar in 1933, Hoover becomes increasingly vengeful against those who challenge his reputation and the Bureau's.

He uses covert listening devices to collect compromising information which he uses to blackmail key political figures over the years, including President Franklin D. Roosevelt and Attorney General Robert F. Kennedy, protecting his position and increasing the Bureau's power.

He starts an illegal counterintelligence program to counter what he perceives as a new wave of radicals, culminating in his unsuccessful attempt to blackmail Martin Luther King Jr. into declining the Nobel Peace Prize in 1964 via the FBI–King suicide letter.

Gunner Wright and David A. Cooper appear briefly as future Presidents Dwight D. Eisenhower and Franklin D. Roosevelt, respectively, in the aftermath of the assassination attempt on A. Mitchell Palmer.

[5] Warner Bros. Pictures wanted to keep the budget down, so producers Grazer and Robert Lorenz brought in Clint Eastwood, known for his efficient filmmaking, to direct and co-produce.

The website's critical consensus reads, "Leonardo DiCaprio gives a predictable powerhouse performance, but J. Edgar stumbles in all other departments: cheesy makeup, poor lighting, confusing narrative, and humdrum storytelling.

[10] Todd McCarthy of The Hollywood Reporter gave the film a positive review, writing, "This surprising collaboration between director Clint Eastwood and Milk screenwriter Dustin Lance Black tackles its trickiest challenges with plausibility and good sense, while serving up a simmeringly caustic view of its controversial subject's behavior, public and private.

"[12] J. Hoberman of The Village Voice wrote: "Although hardly flawless, Eastwood's biopic is his richest, most ambitious movie since Letters from Iwo Jima and Flags of Our Fathers.

"[13] Peter Debruge of Variety gave the film a mixed review: "Any movie in which the longtime FBI honcho features as the central character must supply some insight into what made him tick, or suffer from the reality that the Bureau's exploits were far more interesting than the bureaucrat who ran it – a dilemma J. Edgar never rises above.