Early years and political rise
McMullen attended after elementary school until 1896, the University of Nebraska and then the George Washington University, where in 1899 he made his law degree. Then he opened in Wymore, Nebraska, a law firm. McMullen was a member of the Republican Party and was elected in 1904 and 1906 in the House of Representatives from Nebraska. In addition, he was still mayor of Wymore. In 1916 he made the move into the state Senate. In 1920, he competed unsuccessfully for the Republican nomination for the gubernatorial elections. Four years later he made then not only the nomination, but also the victory in the actual election for governor, in which he prevailed with 51:41 percent of the vote to Democrat John N. Norton.
Governor of Nebraska
McMullen's tenure as governor began on January 8, 1925; after a re-election in 1926 he was able to officiate a total of four years. During his reign, the economy flourished. The budget deficit was reduced and agriculture, particularly irrigation projects, was promoted. Given the ever-growing volume of traffic were the roads and highways of the Staatesausgebaut. McMullen was also a supporter of the Prohibition Act.
After his two terms McMullen remained politically active. In 1932 he became head of the post office in Beatrice. In 1928 and 1932 he was a delegate to the Republican National Conventions relevant where in both cases Herbert Hoover was nominated as a presidential candidate. In 1940, he competed unsuccessfully for a seat in the U.S. Senate. He had already failed in the primaries. Adam McMullen died in early March 1959, and was buried in Wymore. He was married to Cora Greenwood.