Throughout hisCAREER, Brennan was frequently called upon to play characters considerably older than he was in real life. 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 special dental prosthesis which made him appear to have rotting and broken teeth.
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 extremely diverse, he is probably best remembered for his portrayals in movie westerns, such as Judge Roy Bean in The Westerner, trail hand Nadine Groot in Red River and Deputy Stumpy in Rio Bravo, the last twoDIRECTED by Howard Hawks. He was the first actorTO WIN three Academy Awards and remains the only person to have won three Best Supporting Actor awards. However, even 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 extremely popular with the Union of Film Extras, and since their numbers were overwhelming, he won each time he was nominated. Though he was never thought undeserving of these awards, his thirdWIN led to the disenfranchisement of the Extras Union from Oscar voting.
Brennan was married to the former Ruth Wells (December 8, 1897 – January 12, 1997), whom he married in 1920. The Brennans had a daughter, Ruth Caroline, and two sons, Arthur 'Mike' and Andy.
Brennan bought the 12,000-acre Lightning Creek Ranch 20 miles south of Joseph, Oregon, in 1940. Brennan built the Indian Lodge Motel, a movie theater and a variety store in Joseph, and continued going there between film roles until his death in 1974 at age 80. Some of Brennan's family continue to live in the area.
Joining Brennan and Eisley 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, 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 anti-war and civil rights movements were being aided by overseas Communists from Soviet Union through their support of local, homegrown 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 like Watts and Newark and demonstrations in Birmingham, Alabama—had been the result of otherwise content "Negroes" being stirred up by a small number of "troublemakers" with anti-American agendas. He reportedly expressed glee on the set of his final seriesThe Guns of Will Sonnett upon hearing the news that Martin Luther King, Jr., had been assassinated. In 1972, he supported the presidential campaign of conservative California Congressman John Schmitz over that of Richard Nixon, whom he believed was too moderate.