[4] While working as a bank clerk, he enlisted in the U.S. Army and served as a private with the 101st Field Artillery Regiment in France for two years[5] during World War I.

[2][6] "While there, he suffered an injury to his vocal cords from exposure to mustard gas that left him with his screen trademark: a distinctively reedy, high-pitched voice that became a favorite for celebrity impersonators for decades.

[4] Finding himself penniless, Brennan began taking parts as an extra in films at Universal Studios in 1925, starting at $7.50 a day, equal to $130 today.

[9] His early appearances included Webs of Steel (1925), Lorraine of the Lions (1925) and The Calgary Stampede (1925), a Hoot Gibson Western.

His parts tended to remain small, however: A House Divided (1931) for director William Wyler, Scratch-As-Catch-Can (1931, a Bobby Clark short directed by Mark Sandrich), and Texas Cyclone (1931, a Tim McCoy Western featuring a young John Wayne).

In 1932 Brennan was in Law and Order (1932) with Walter Huston, The Impatient Maiden (1932) for James Whale, The Airmail Mystery (1932, a serial) and Scandal for Sale (1932).

Brennan was in Hello Trouble (1932) with Buck Jones, Speed Madness (1932), Miss Pinkerton (1932) with Joan Bennett, Cornered (1932) with McCoy, The Iceman's Ball (1932, another short for Sandrich), Fighting for Justice (1932) with McCoy, The Fourth Horseman (1932) with Tom Mix, The All American (1932), Once in a Lifetime (1932), Strange Justice (1932), Women Won't Tell (1932) for Richard Thorpe, Afraid to Talk (1932) and Manhattan Tower (1932).

Brennan was in Sensation Hunters (1933) for Charles Vidor, Man of Action (1933) with McCoy, Parachute Jumper (1933), Goldie Gets Along (1933), Girl Missing (1933), Rustlers' Roundup (1933) with Mix, The Cohens and Kellys in Trouble (1933) for director George Stevens, Lucky Dog (1933) and The Big Cage (1933).

James Whale gave him a bit part in The Invisible Man (1933) and he could be seen in King for a Night (1933), Fugitive Lovers (1933), Cross Country Cruise (1934), Beloved (1934), You Can't Buy Everything (1934), Paradise Valley (1934), Radio Dough (1934, a short), The Poor Rich (1934), The Crosby Murder Case (1934), George White's Scandals (1934), Good Girl (1934), Riptide (1934), Uncertain Lady (1934), I'll Tell the World (1934) and Fishing for Trouble (1934, a short).

Brennan was in the Three Stooges short Woman Haters (1934), then did Half a Sinner (1934), The Life of Vergie Winters (1934), Murder on the Runaway Train (1934), Whom the Gods Destroy (1934), Gentlemen of Polish (1934, a short), Death on the Diamond (1934), Great Expectations (1934), Luck of the Game (1934), Tailspin Tommy (1934, a serial), There's Always Tomorrow (1934) and Cheating Cheaters (1934).

A break for Brennan came when he was cast in The Wedding Night (1935), produced by Sam Goldwyn, alongside Gary Cooper (it was actually their second film together).

Brennan's parts remained small in Party Wire (1935), Spring Tonic (1935), The Gay Lady (1935), Man on the Flying Trapeze (1935) and Welcome Home (1935).

Brennan finally earned significant roles with a decent part in Goldwyn's Barbary Coast (1935), directed by Howard Hawks and an uncredited William Wyler.

He had a small role in These Three (1936) with Wyler and a larger one in Walter Wanger's The Moon's Our Home (1936) and Fury (1936), directed by Fritz Lang.

[18] Brennan had one of his best roles in Goldwyn's The Westerner (1940), playing the villainous Judge Roy Bean opposite Gary Cooper.

(1941) and then Cooper again in Frank Capra's Meet John Doe (1941) and Hawks' Sergeant York (1941), a role that earned Brennan a fourth Oscar nomination.

Brennan returned as a villain as Old Man Clanton in John Ford's My Darling Clementine (1946), opposite Henry Fonda.

Brennan focused on Westerns: Singing Guns (1950), A Ticket to Tomahawk (1950), Curtain Call at Cactus Creek (1950), The Showdown (1950), Surrender (1950), Along the Great Divide (1951), Best of the Badmen (1951) and Return of the Texan (1952).

Brennan was in Sea of Lost Ships (1953) with John Derek, Drums Across the River (1954) with Audie Murphy, The Far Country (1954) with James Stewart and Four Guns to the Border (1954) with Rory Calhoun.

Brennan appeared in films such as Glory (1956), Come Next Spring (1956) and Batjac's Good-bye My Lady (1956) with 14-year-old Brandon deWilde, with whom he recorded The Stories of Mark Twain that same year.

[25] Brennan had resisted overtures to star in a regular TV series but relented for The Real McCoys, a sitcom about a poor West Virginia family that relocated to a farm in Southern California.

[27] Brennan continued to appear in films and other TV shows during the series' run such as Colgate Theatre and another Howard Hawks picture, Rio Bravo (1959), supporting John Wayne and Dean Martin.

Brennan starred as the wealthy executive Walter Andrews in the short-lived 1964–1965 ABC series The Tycoon, with Van Williams.

[32] This was Brennan's last television series as a member of the permanent cast, although he did make a number of appearances on Alias Smith and Jones.

He built the Indian Lodge Motel, a movie theater, and a variety store in Joseph, and continued retreating to the ranch between film roles until his death.

The gathering, hosted by Anthony Eisley, sought to flood Congress with letters in support of school prayer following two decisions by the Supreme Court in 1962 and 1963 that had invalidated the practice of mandatory prayer in public schools, which the court ruled to have conflicted with the Establishment Clause of the First Amendment to the United States Constitution.

[38] According to his biographer Carl Rollyson, Brennan was fiercely opposed to communism and reportedly branded people as communists if they supported John F. Kennedy.

Rollyson also reported that Brennan's home "included a bunker stocked with weapons and food in anticipation of a Soviet invasion."

[39] Everett Greenbaum, who wrote 32 episodes of The Real McCoys, described Brennan as a bigot who frequently uttered racist remarks.

[40] A staunch conservative Republican and a member of the Motion Picture Alliance for the Preservation of American Ideals, Brennan supported Barry Goldwater in the 1964 United States presidential election because Goldwater had voted against the Civil Rights Act of 1964, but Brennan later endorsed George Wallace in 1968, believing that Richard Nixon was too liberal for a Republican.