Owen (he preferred to be known by his middle name) Brewster was born in Dexter, the son of William Edmund Brewster, a member of the Maine House of Representatives, and Carrie S. Bridges. He graduated summa cum laude from Bowdoin College in 1909, a member of the Phi Beta Kappa Society and Delta Kappa Epsilon. From 1909 to 1910, Brewster was the principal of Castine High School, and then attended Harvard Law School, graduating in 1913.

In 1915, he married Dorothy Foss; from 1915 to 1923, he was a member of the Portland School Committee. From 1914 to 1925, Brewster was a lawyer for the Chapman and Brewster law firm in Portland. He was elected to a two-year term as a member of the State House (1917-18), but resigned to enlist in the U.S. Army (Third Infantry unit of the Maine National Guard) when the nation entered World War I. Brewster served successively as private, second lieutenant, captain, and regimental adjutant, and returned to the State House after the war ended. He continued to be a House member from 1921 to 1922, when he was elected to the State Senate.

Brewster served in the State Senate until 1925, when he became the Governor of Maine. Brewster served two terms as Governor, leaving office in 1929, losing the Republican nomination for a third term. In 1932, he was defeated for a seat in the U.S. House of Representatives, but was elected from the Third District in 1935 and served until 1941, when he went on to the U.S. Senate. Brewster was reelected in 1946.

During his time in Congress, Brewster worked on legislation to provide old-age pensions (the forerunner of Social Security) although he was a prominent opponent of welfare and spending programs in President Roosevelt's New Deal. As Senator, Brewster sat on several Senate committees, notably the Special Senate Committee to Investigate the National Defense Program (the Truman Committee), and the Joint Committee to Investigate the Pearl Harbor Attack. At the time these were very high profile and Brewster's work on those committees did much to raise his profile in Washington.

In the Senate, he was good friends with Joseph McCarthy of Wisconsin. His association with McCarthyism eroded Brewster's political support as McCarthy's anti-communist excesses became increasingly unpopular. His alleged association with the Ku Klux Klan also cost him support in liberal Republican circles.

But Brewster came to national attention due to his opposition to the commercial interests of Howard Hughes, America's wealthiest man at the time. Brewster was chairman of a special Senate committee investigating defense procurement during World War II. He claimed concern that Hughes had received $40 million from the Defense Department without actually delivering the aircraft he had contracted to provide, but Brewster may have had an ulterior motive.

Hughes aggressively combated the inquirer, alleging corruption. Memoirs by Hughes right-hand man Noah Dietrich and syndicated newspaper columnist Jack Anderson each sketched Brewster as, in Dietrich's words, "an errand boy for Juan Trippe and Pan American World Airways," who pushed for legislation that would give Pan Am the single-carrier international air monopoly for the U.S. The Martin Scorsese movie The Aviator portrays Brewster (played by Alan Alda) similarly, as corrupt and in the pocket of Pan Am, the rival of Hughes' TWA. Hughes spread rumors about Brewster's close association with Pan Am, alleging that he received free flights and hospitality in return for legislation such as his bill to withdraw government approval for TWA flights across the Atlantic.

In a Senate hearing that electrified the nation, Hughes repeated his accusations that Brewster had promised an end to the Senate inquiry if Hughes would agree to merging TWA with Pan Am. (Dietrich wrote that Hughes, in a bid to stall for time before the hearing, went so far as to launch negotiations with Trippe about such a merger.) In response, Brewster, stung by the allegations, stood aside from chairing the inquiry and became instead a witness before the committee---which also allowed Hughes to question Brewster directly. Brewster denied Hughes' allegations and made several counter-claims, but by the time the hearing ended Brewster's reputation suffered greatly from the incident. Paradoxically, Hughes---for all his wealth and elusiveness---came across as what Dietrich described as the little guy who "fought City Hall---and won."

In 1952 Hughes worked hard to ensure Brewster's political demise, persuading the then Governor of Maine, Frederick G. Payne, to challenge him for the Republican Senate nomination. Armed with practically unlimited campaign funds from Hughes, Payne challenged Brewster's connection with McCarthyism, racist groups and also took up Hughes' claims that Brewster was corrupt. This led to the unusual defeat of an incumbent Senator in his own primary; Payne would only last one term, being defeated by Ed Muskie in 1958.

In his retirement Brewster continued active involvement in many conservative organizations. Brewster was a Christian Scientist. He was a member of the American Bar Association, Grange, the American Legion, the Freemasons, the Elks, the Odd Fellows, and Delta Kappa Epsilon. He denied being a member of the Ku Klux Klan, although he received their support.

Brewster died unexpectedly of cancer in 1961 in Brookline, Massachusetts. He was buried at Mount Pleasant Cemetery in Dexter, Maine where his home was converted to the Brewster Inn, a bed and breakfast.