Garner was born on November 22, 1868, in a log cabin near Detroit in Red River County to John Nance Garner II and his wife, Sarah Guest Garner. The mud-chinked log cabin that Garner was born in no longer exists but the house that he grew up in survives and is located at 260 South Main Street in Detroit, Texas. It is a large, white, two-story house. Garner attended Vanderbilt University in Nashville, Tennessee, for one semester before dropping out and returning home. He eventually studied law, was admitted to the bar in 1890, and began practice in Uvalde, Uvalde County, Texas.

In 1893, Garner entered politics, running for county judge of Uvalde County. (Although the county judge in Texas is now primarily the chief administrative officer of a county, comparable to the mayor of a city, the office is a judicial position, and the county judge sits in small civil cases, misdemeanor Criminal cases, and probate cases.) At that time, Democrats entirely dominated politics in Texas, and the Democratic nomination for an office was tantamount to election. Thus the Democratic primary election was the real election, with the general election being a formality.

Garner was opposed in the county judge primary by a woman—Mariette Rheiner, a rancher's daughter. Two years later, on November 25, 1895, she married Garner in Sabinal, Texas. They had one child, Tully Charles Garner.

Garner was elected county judge and served until 1896.

Garner was elected to the Texas House of Representatives in 1898, and re-elected in 1900. During his Service, the legislature selected a state flower for Texas. Garner fervently supported the prickly pear cactus for the honor, and thus earned the nickname "Cactus Jack". (The Bluebonnet was chosen.)

In 1901 Garner voted for the poll tax, a measure passed by the Democratic-dominated legislature to make voter registration more difficult and reduce the number of black, minority, and poor white voters on the voting rolls. This disfranchised most minority voters until the 1960s, and ended challenges to Democratic power; Texas became in effect a one-party state.

In 1902, Garner was elected to the United States House of Representatives from the newly created 15th congressional district, a narrow strip reaching south to include tens of thousands of square miles of rural areas. He was elected from the district 14 subsequent times, serving until 1933. His wife was paid and worked as his private secretary during this period.

Garner was chosen to serve as minority floor leader for the Democrats in 1929, and in 1931 as Speaker of the United States House of Representatives, when the Democrats became the majority.

Garner was re-elected to the 73rd Congress on November 8, 1932, and on the same day was elected Vice President of the United States. He was the second man, Schuyler Colfax being the first, to serve as both Speaker of the House and President of the Senate. Garner was re-elected Vice President with Roosevelt in 1936, serving in that office in total from March 4, 1933, to January 20, 1941.

During Roosevelt's second term, Garner's previously warm relationship with the President quickly soured, as Garner disagreed sharply with him on a wide range of important issues. Garner supported federal intervention to break up the Flint sit-down strike, supported a balanced federal budget, opposed the Judiciary Reorganization Bill of 1937 to "pack" the Supreme Court with additional judges, and opposed executive interference with the internal Business of the Congress.

During 1938 and 1939, numerous Democratic party Leaders urged Garner to run for President in 1940. Garner identified as the champion of the traditional Democratic Party establishment, which often clashed with supporters of Roosevelt's New Deal. The Gallup poll showed that Garner was the favorite among Democratic voters, based on the assumption that Roosevelt would defer to the longstanding two-term tradition and not run for a third term. Time characterized him on April 15, 1940:

Garner left office on January 20, 1941, ending a 46-year career in public life. He retired to his home in Uvalde for the last 26 years of his life, where he managed his extensive real estate holdings, spent time with his great-grandchildren, and fished. Throughout his retirement, he was consulted by active Democratic politicians and was especially close to Roosevelt's successor, Harry S. Truman.

On the morning of Garner's 95th birthday, November 22, 1963, President John F. Kennedy called to wish him a happy birthday. This was just hours before Kennedy's assassination. Dan Rather states that he visited the Garner ranch that morning to film an interview with Garner, where Miss Texas Wool was in attendance, and that he then flew back to Dallas from Uvalde to deposit the film at then-CBS affiliate KRLD-TV (now Fox owned-and-operated KDFW-TV).

Garner died on November 7, 1967, at the age of 98 years and 350 days, 15 days before his 99th birthday. He is interred in Uvalde Cemetery. Garner is, as of 2018, the longest-lived vice President or President in United States history, a record which was previously held by Benjamin Harrison's vice President, Levi P. Morton (who died in 1920 on his 96th birthday). He is one of the few people to live during the presidencies of both Presidents Johnson—Andrew and Lyndon.