Thomas was the oldest of six children, born November 20, 1884 in Marion, Ohio to Emma Williams Mattoon and Weddington Evans Thomas, a Presbyterian minister. Thomas had an uneventful midwestern childhood and adolescence, helping to put himself through Marion High School as a paper carrier for Warren G. Harding's Marion Daily Star. Like other paper carriers, he reported directly to Florence Kling Harding. "No pennies ever escaped her," said Thomas. The summer after he graduated from high school his father accepted a pastorate at Lewisburg, Pennsylvania, which allowed Norman to attend Bucknell University. He left Bucknell after one year to attend Princeton University, the beneficiary of the largesse of a wealthy uncle by marriage. Thomas graduated magna cum laude from Princeton University in 1905.

After some settlement work and a trip around the world, Thomas decided to follow in his father's footsteps and enrolled in Union Theological Seminary. He graduated from the seminary and was ordained as a Presbyterian minister in 1911. After assisting the Rev. Henry Van Dyke at the fashionable Brick Presbyterian Church on Manhattan's Fifth Avenue, Thomas was appointed as pastor for the East Harlem Presbyterian Church, ministering to Italian-American Protestants. Union Theological Seminary had been, at that time, a center of the Social Gospel movement and liberal politics, and as a minister, Thomas preached against American participation in the First World War. This pacifist stance led to his being shunned by many of his fellow alumni from Princeton, and opposed by some of the leadership of the Presbyterian Church in New York. When church funding of the American Parish's social programs was stopped, Thomas resigned his pastorate. Despite this resignation of his position, Thomas did not formally leave the ministry until 1931, after his mother's death.

It was Thomas' position as a conscientious objector which drew him to the Socialist Party of America (SPA), a staunchly antimilitarist organization. When SPA leader Morris Hillquit made his campaign for Mayor of New York in 1917 on an anti-war platform, Thomas wrote to him expressing his good wishes. To his surprise, HIllquit wrote back, encouraging the young minister to work for his campaign, which Thomas energetically did. Soon thereafter he himself joined the Socialist Party. Despite his membership in the Marxist SPA, Thomas was never himself an orthodox Marxist, instead favoring a Christian socialist orientation.

Thomas was the secretary (then an unpaid position) of the pacifist Fellowship of Reconciliation even before the war. When the organization started a magazine called The World Tomorrow in January 1918, Thomas was employed as its paid editor. Together with his co-thinker Devere Allen, Thomas helped to make The World Tomorrow the leading voice of liberal Christian social activism of its day. In 1921, Thomas moved to secular journalism, when he was employed as associate editor of The Nation magazine.

In 1922 Thomas became co-director of the League for Industrial Democracy. Later, he was one of the founders of the National Civil Liberties Bureau (the precursor of the American Civil Liberties Union).

Thomas ran for office four times in quick succession on the Socialist ticket ... for Governor of New York in 1924, for Mayor of New York in 1925, for New York State Senate in 1926, for Alderman in 1927. Following Eugene Debs' death in 1926, there was a leadership vacuum in the Socialist Party. Neither of the party's two top political leaders ... Victor L. Berger and Hillquit ... were eligible to run for President of the United States by virtue of their foreign birth. The third main figure, Daniel Hoan was occupied as Mayor of Milwaukee, Wisconsin. Down to approximately 8,000 dues paying members, the Socialist Party's options were limited, and the little known minister from New York with oratorial skills and a pedigree in the movement became the choice of the 1928 National Convention of the Socialist Party as its standard bearer. In 1934, he ran for U.S. Senator from New York and polled almost 200,000 votes, then the second best result of Socialist candidates in New York state elections, only Charles P. Steinmetz polled more votes, almost 300,000 in 1922 for State Engineer.

The 1928 campaign marked the first of six consecutive campaigns of Thomas running as the Presidential nominee of the Socialist Party. As an articulate and engaging spokesman for democratic socialism, Thomas' influence was considerably greater than that of the typical perennial candidate. Although socialism was viewed as an unsavory form of political thought by most middle-class Americans, the well-educated Thomas -- who often wore three-piece suits -- looked like and talked like a president and gained grudging admiration.

Thomas frequently spoke on the difference between socialism and Communism, explaining the differences between the movement he represented and that of revolutionary Marxism. His early admiration for the Russian Revolution subsequently turned into energetic anti-Communism. (The revolutionaries thought him no better; Leon Trotsky, on more than one occasion, levelled high-profile criticism at Thomas.) He wrote several books, among them his passionate defense of World War I conscientious objectors, Is Conscience a Crime?, and his statement of the 1960s social democratic consensus, Socialism Re-examined.

Thomas failed to isolate himself from the rough and tumble internal factional politics of the Socialist Party, as his predecessor Debs had been able to do. At the 1932 Milwaukee Convention, Thomas and his radical pacifist allies in the party joined forces with constructive socialists from Wisconsin and a faction of young Marxist intellectuals called the "Militants" in backing a challenger to National Chairman Morris Hillquit. While Hillquit and his cohort retained control of the organization at this time, this action earned the lasting enmity of Hillquit's New York-based allies of the so-called "Old Guard". The diplomatic party peacemaker Hillquit died of tuberculosis the following year, lessening the stability of his faction.

At the 1934 Convention, Thomas' connection with the Militants was deepened when he backed a radical Declaration of Principles authored by his long-time associate from the radical pacifist journal The World Tomorrow, Devere Allen. The Militants swept to majority control of the party's governing National Executive Committee at this gathering, and the Old Guard retreated to their New York fortress and formalized their factional organization as the Committee for the Preservation of the Socialist Party, complete with a shadow Provisional Executive Committee and an office in New York City.

Although Thomas himself favored work to establish a broad Farmer-Labor Party upon the model of the Canadian Cooperative Commonwealth Federation, he nonetheless remained supportive of the Militants and their vision of an "all-inclusive party," which welcomed members of dissident communist organizations (including Lovestoneites and Trotskyists) and worked together with the Communist Party USA in joint Popular Front activities. The party descended into a maelstrom of factionalism in the interval, with the New York Old Guard leaving to establish themselves as the Social Democratic Federation of America, taking with them control of party property, such as the Yiddish-language Jewish Daily Forward, the English-language New Leader, the Rand School of Social Science, and the party's summer camp in Pennsylvania. The party was left in dire financial circumstances. As the social democratic Marxists of the Old Guard were expelled and left the SP in 1936, revolutionary Marxists from the Workers Party of the United States were admitted en masse. Disagreements among the Militant faction led it to shatter into three rival groups, a Right Wing headed by Jack Altman, a Center group called "Clarity" headed by Herbert Zam and Gus Tyler, and a Trotskyist revolutionary Left Wing faction called the "Appeal" group after the name of their factional newspaper.

In 1937 Thomas returned from Europe determined to restore order in the Socialist Party. He and his followers in the party teamed up with the Clarity majority of the National Executive Committee and gave the green light to the New York Right Wing to expel the Appeal faction from the organization. These expulsions led to the departure of virtually the whole of the party's youth section, who affiliated to the new Trotskyist Socialist Workers Party. Demoralization set in and the Socialist Party withered, its membership level below the lowest nadir of 1928.

Thomas was initially as outspoken in opposing the Second World War as he was with regard to the First World War. Upon returning from a European tour in 1937, he formed the Keep America Out of War Congress and spoke against war, thereby sharing a platform with the America First Committee. However, after the United States was attacked by the Japanese at Pearl Harbor, his stance changed to support for US involvement, and later wrote self-critically for having "overemphasized both the sense in which it was a continuance of World War I and the capacity of nonfascist Europe to resist the Nazis.".

Thomas was one of the few public figures to oppose President Franklin Roosevelt's (D) internment of Japanese Americans following the attack on Pearl Harbor. Thomas accused the ACLU of "dereliction of duty" when the organization supported the internment. Thomas also campaigned against racial segregation, environmental depletion, anti-labor laws and practices, and in favor of opening the United States to Jewish victims of Nazi persecution in the 1930s.

Thomas was an early proponent of birth control. The birth-control advocate Margaret Sanger recruited him to write "Some Objections to Birth Control Considered" in Religious and Ethical Aspects of Birth Control, edited and published by Sanger in 1926. Thomas accused the Roman Catholic Church of hypocritical opinions on sex, such as requiring priests to be celibate and maintaining that lay people should only have sex to reproduce. "This doctrine of unrestricted procreation is strangely inconsistent on the lips of men who practice celibacy and preach continence."

Thomas also deplored the secular objection to birth control because it originated from "racial and national" group-think. "The white race, we are told, our own nation ... whatever that nation may be ... is endangered by practicing birth control. Birth control is something like disarmament ... a good thing if effected by international agreement, but otherwise dangerous to us in both a military and economic sense. If we are not to be overwhelmed by the 'rising tide of color' we must breed against the world. If our nation is to survive, it must have more cannon and more babies as prospective food for the cannon."

Thomas was also very critical of Zionism and of Israel's policies towards the Arabs in the postwar years (especially after the Suez Crisis) and often collaborated with the American Council for Judaism.

After 1945 Thomas sought to make the non-Communist left the vanguard of social reform, in collaboration with labor leaders like Walter Reuther. He championed many seemingly unrelated progressive causes, while leaving unstated the essence of his political and economic philosophy.

In 1961, Thomas released an album The Minority Party in America: Featuring an Interview with Norman Thomas, on Folkways Records, which focused on the role of the third party.

Thomas' 80th birthday in 1964 was marked by a well-publicized gala at the Hotel Astor in Manhattan. At the event Thomas called for a cease-fire in Vietnam and read birthday telegrams from Hubert Humphrey, Earl Warren, and Dr. Martin Luther King, Jr. He also received a check for $17,500 in donations from supporters. "It won't last long," he said of the check, "because every organization I'm connected with is going bankrupt."

In 1966, he was chosen by conservative editor William F. Buckley, Jr to be the first guest on Buckley's new television interview show Firing Line.

Thomas died on December 19, 1968.

The Norman Thomas High School in Manhattan (which is now under threat of closure) and the Norman Thomas '05 Library at Princeton University's Forbes college are named after him. He is also the grandfather of Newsweek columnist Evan Thomas.

A plaque in the Norman Thomas '05 Library reads: Norman M. Thomas, class of 1905. "I am not the champion of lost causes, but the champion of causes not yet won."