Steffens began his career as a journalist at the New York Evening Post. He later became an editor of McClure's magazine, where he became part of a celebrated muckraking trio, along with Ida Tarbell and Ray Stannard Baker. He specialized in investigating government and political corruption, and two collections of his articles were published as The Shame of the Cities (1902) and The Struggle for Self - Government (1906). He also wrote The Traitor State, which criticized New Jersey for patronizing incorporation. In 1906, he left McClure's, along with Tarbell and Baker, to form The American Magazine.

In The Shame of the Cities, Steffens sought to bring about political reform in urban America by appealing to the emotions of Americans. He tried to provoke outrage with examples of corrupt governments throughout urban America.

In 1910 he covered the Mexican Revolution and began to see revolution as preferable to reform. In March 1919, he accompanied William C. Bullitt, a low level State Department official, on a three week visit to the Soviet Union and witnessed the "confusing and difficult" process of a society in the process of revolutionary change. He wrote that "Soviet Russia was a revolutionary government with an evolutionary plan", enduring "a temporary condition of evil, which is made tolerable by hope and a plan."

Upon his return, he promoted his view of the Soviet Revolution and in the course of campaigning for U.S. food aid for Russia made his famous remark about the new Soviet society: "I have seen the future, and it works", a phrase he repeated often with many variations. His enthusiasm for communism soured by the time his memoirs appeared in 1931.

He was a member of the California Writers Project, a New Deal program.

He died on August 9, 1936, in Carmel, California.