Brennan was born in Lynn, Massachusetts, less than two miles from his family's home in Swampscott, Massachusetts. He was the second of three children born to Margaret Elizabeth (née Flanagan; June 4, 1869 in Charlestown, Massachusetts – February 1, 1955 in Pasadena, California) and William John Brennan (September 2, 1868 in Malden, Massachusetts – August 17, 1936 in Pasadena, California). His parents were both of Irish descent. His father was an engineer and inventor, and young Brennan also studied engineering at Rindge Technical High School in Cambridge, Massachusetts.
While in school, Brennan became interested in acting. He began to perform in vaudeville at the age of 15. 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 during World War I. After the war, he moved to Guatemala and grew pineapples before returning to the U.S. and settling in Los Angeles. During the early 1920s, he made a fortune in the real estate market, but lost most of his money during the 1925 real estate slump.
Finding himself penniless, Brennan began taking parts as an extra in films in 1925 and then bit parts in as many films as he could, including Texas Cyclone and Two Fisted Law with another newcomer to Hollywood, John Wayne. Brennan also had bit parts in The Invisible Man (1933), Girl Missing (1933), the Three Stooges short Woman Haters (1934), and Bride of Frankenstein (1935), in which he had a brief speaking part and also worked as a stuntman. In the 1930s, he began appearing in higher-quality films and received more substantial roles as his talent was recognized. This culminated with his receiving the first Academy Award for Best Supporting Actor for his role as Swan Bostrom in the period film Come and Get It (1936). Two years later, he portrayed town drunk and accused murderer Muff Potter in The Adventures of Tom Sawyer.
Throughout his career, Brennan was frequently called upon to play characters considerably older than he was. The loss of many teeth in a 1932 accident, rapidly thinning hair, thin build, and unusual vocal intonations all made him seem older than he really was. He used these features to great effect. In many of his film roles, Brennan wore dentures; in Northwest Passage – a film set in the late 18th century – he wore a dental prosthesis which made him appear to have rotting and broken teeth. Brennan played the top-billed lead in Swamp Water (1941), the first American film by the director Jean Renoir, a drama also featuring Walter Huston and starring Dana Andrews.
In Sergeant York (1941), he played a sympathetic preacher and dry-goods store owner who advised the title character, played by Gary Cooper. Brennan and Cooper appeared in six films together. In 1942, he played the reporter Sam Blake, who befriended and encouraged Lou Gehrig (played by Cooper) in Pride of the Yankees. He was particularly skilled in playing the sidekick of the protagonist or the "grumpy old man" in films such as To Have and Have Not (1944), the Humphrey Bogart vehicle which introduced Lauren Bacall. Though he was hardly ever cast as the villain, notable exceptions were his roles as Judge Roy Bean in The Westerner (1940) with Gary Cooper, for which he won his third Academy Award for Best Supporting Actor; Old Man Clanton in My Darling Clementine (1946), opposite Henry Fonda; and the murderous Colonel Jeb Hawkins in the James Stewart episode of the Cinerama production How the West Was Won (1962).
From 1957 to 1963, he starred in the ABC television series The Real McCoys, a sitcom about a poor West Virginia family that relocated to a farm in Southern California. After five years on ABC, The Real McCoys switched to CBS for a final season. Brennan joined with the series creator, Irving Pincus, to form Brennan-Westgate Productions. The series was co-produced with Danny Thomas's Marterto Productions. It also featured Richard Crenna, Kathleen Nolan, Lydia Reed, and Michael Winkelman.
Brennan appeared in several other movies and television programs, usually as an eccentric old-timer or prospector. Prior to the launching of The Real McCoys, he appeared as himself as a musical judge in the 1953–1954 ABC series Jukebox Jury. On May 30, 1957, he guest-starred on NBC's The Ford Show, Starring Tennessee Ernie Ford. He also made a few recordings, the most popular being "Old Rivers", about an old farmer and his mule, which was released as a single in 1962 by Liberty Records with "The Epic Ride of John H. Glenn" on the flip side. "Old Rivers" peaked at number five in the U.S. Billboard chart. In his music, he sometimes worked with Allen "Puddler" Harris, a Louisiana native who was a member of the original Ricky Nelson Band. Brennan appeared as an extremely cantankerous sidekick with John Wayne, Dean Martin and Ricky Nelson in Howard Hawks's 1959 Western Rio Bravo, and also co-starred with James Garner a decade later in Support Your Local Sheriff!, playing the ruthless head of the villainous Danby family.
Brennan starred as the wealthy executive Walter Andrews in the short-lived 1964–1965 ABC series The Tycoon, with Van Williams. In 1967, he starred in another ABC series, The Guns of Will Sonnett, as an older man in search of his gunfighter son, James Sonnett, with his grandson, Jeff, played by Dack Rambo. After the series went off the air in 1969, Brennan continued working in both television and feature films. He received top billing over Pat O'Brien in the TV movie The Over-the-Hill Gang (1969) and Fred Astaire in The Over-the-Hill Gang Rides Again the following year. From 1970 to 1971, he was a regular on the CBS sitcom To Rome with Love, with John Forsythe. This was Brennan's last television series as a member of the permanent cast.
In 1920, Brennan married Ruth Caroline Wells (December 8, 1897 – January 12, 1997). They had a daughter, Ruth Caroline Brennan Lademan (September 22, 1924 – October 27, 2004). Lademan's husband, Dixon McCully Lademan (1924–2003), was a captain in the U.S. Navy in World War II, the Korean War, and the Vietnam War.
The two Brennan sons are both apparently living. Arthur Mike Brennan (born 1921) and his wife, Florence Irene Brennan (1925–2003), lived in Joseph, Oregon. Brennan's other son is Walter Andrew "Andy" Brennan Jr. (born 1923). In 1940, Brennan purchased the 12,000-acre Lightning Creek Ranch, 20 miles south of Joseph. He built the Indian Lodge Motel, a movie theater, and a variety store in Joseph, and continued going there between film roles until his death. Some members of his family continue to live in the area.
Brennan was politically conservative. In 1963 and 1964, he joined actors William Lundigan, Chill Wills, and Efrem Zimbalist, Jr. in making appearances on behalf of U.S. Senator Barry M. Goldwater, the Republican nominee in the campaign against President Lyndon B. Johnson.
In 1964, Brennan spoke at "Project Prayer", a rally attended by 2,500 at the Shrine Auditorium in Los Angeles. The gathering, which was hosted by Anthony Eisley, sought to flood Congress with letters in support of school prayer, following two decisions of the Supreme Court in 1962 and 1963 which struck down the practice of enforced prayer in public schools as being in conflict with the Establishment Clause of the First Amendment to the United States Constitution. Also at the rally were Rhonda Fleming, Lloyd Nolan, Dale Evans, Pat Boone, and Gloria Swanson.
Brennan, a Roman Catholic, did not publicize his own religious affiliation, but declared, "I'm too old not to be a religious fella. [...] It appears we are losing something a lot of people made a lot of sacrifices for." During the 1960s, he became convinced that the antiwar and civil rights movements were being aided by overseas Communists from the Soviet Union through their support of local communist sympathizers and agitators, and said as much in interviews. He told reporters that he believed the civil rights movement – in particular, the riots in places such as Watts and Newark and demonstrations in Birmingham – had been the result of otherwise content "Negroes" being stirred up by a small number of "troublemakers" with anti-American agendas. In 1972, he supported the presidential campaign of conservative California congressman John Schmitz over that of Richard Nixon, whom he believed to be too moderate. He was a member of the John Birch Society.
Brennan died of emphysema at the age of 80 in Oxnard, California. His remains were interred at San Fernando Mission Cemetery in Los Angeles.
Film historians and critics have long regarded Brennan as one of the finest character actors in motion picture history. While the roles he was adept at playing were diverse, he is probably best remembered for his portrayals in Western movies, such as Judge Roy Bean in The Westerner, trail hand Nadine Groot in Red River, and Deputy Stumpy in Rio Bravo. He was the first actor to win three Academy Awards and remains the only person to have won Best Supporting Actor three times. However, he remained somewhat embarrassed as to how he won the awards; in the early years of the Academy Awards, extras were given the right to vote. Brennan was popular with the Union of Film Extras, and since their numbers were overwhelming, he won every time he was nominated. His third win led to the disenfranchisement of the union from Oscar voting. Following this change, Brennan lost his fourth Best Supporting Actor nomination in 1941 for Sergeant York (the award went to Donald Crisp for How Green Was My Valley).
In all, Brennan appeared in more than 230 film and television roles during a career that spanned nearly five decades. For his contributions to the motion picture industry, he has a star on the Hollywood Walk of Fame at 6501 Hollywood Boulevard. In 1970, he was inducted into the Western Performers Hall of Fame at the National Cowboy & Western Heritage Museum in Oklahoma City, where his photograph hangs prominently.The Real McCoys – 224 episodes – Grandpa Amos McCoy (1957–1963)
The Tycoon – 32 episodes – Walter Andrews (1964–1965)
The Guns of Will Sonnett – 50 episodes – Will Sonnett (1967–1969)
The Young Country (TV movie) – Sheriff Matt Fenley (1970)
To Rome with Love – 17 episodes – Andy Pruitt (1970–1971)
Alias Smith and Jones – episode – "The Day They Hanged Kid Curry" – Silky O'Sullivan (1971)
Alias Smith and Jones – episode – "21 Days to Tenstrike" – Gantry (1972)
Alias Smith and Jones – episode – "Don't Get Mad, Get Even" – Silky O'Sullivan (1972)