From the age of 11, Sandburg worked in various occupations—as a barbershop porter, a milk truck driver, a brickyard hand, and a harvester in the Kansas wheat fields. When the Spanish-American War broke out in 1898, he enlisted in the 6th Illinois Infantry. These early years he later described in his autobiography Always the Young Strangers (1953).

From 1910 to 1912 he acted as an organizer for the Social Democratic Party and secretary to the mayor of Milwaukee. Moving to Chicago in 1913, he became an editor of System, a business magazine, and later joined the staff of the Chicago Daily News.

In 1914 a group of his Chicago Poems appeared in Poetry magazine (issued in book form in 1916). In his most famous poem, “Chicago,” he depicted the city as the laughing, lusty, heedless “Hog Butcher, Tool Maker, Stacker of Wheat, Player with Railroads and Freight Handler to the Nation.” Sandburg's poetry made an instant and favourable impression. In Whitmanesque free verse, he eulogized workers: “Pittsburgh, Youngstown, Gary, they make their steel with men” (Smoke and Steel, 1920).

In Good Morning, America (1928) Sandburg seemed to have lost some of his faith in democracy, but from the depths of the Great Depression he wrote a poetic testament to the power of the people to go forward, The People, Yes (1936). The folk songs he sang before delighted audiences were issued in two collections, The American Songbag (1927) and New American Songbag (1950). He wrote the popular biography Abraham Lincoln: The Prairie Years, 2 vol. (1926), and Abraham Lincoln: The War Years, 4 vol. (1939; Pulitzer Prize in history, 1940).

Another biography, Steichen the Photographer, the life of his famous brother-in-law, Edward Steichen, appeared in 1929. In 1948 Sandburg published a long novel, Remembrance Rock, which recapitulates the American experience from Plymouth Rock to World War II. Complete Poems appeared in 1950. He wrote four books for children—Rootabaga Stories (1922); Rootabaga Pigeons (1923); Rootabaga Country (1929); and Potato Face (1930).