Born in New York City, Hal Draper was the son of Jewish parents who changed their children's last names from Dubinsky to shield them from anti-Semitism. He graduated from Boys High School and earned a bachelor's degree from Brooklyn College in 1934, during the Great Depression. During his teenage years he joined the Young People's Socialist League (YPSL), then the youth affiliate of the Socialist Party of America, and became a major leader of the national student movements of the day, which organized against fascism, war, and unemployment.

Draper's political choices, both then and later, were in contrast to those made by his brother Theodore Draper, a Communist Party member in the 1930s who would later be disillusioned with Communism and become a Cold War liberal. Their sister Dorothy (Dora) Draper would marry Jacob Rabkin (1905-2003), one of the intellectual founders of US tax law.

Within the YPSL, Hal Draper was won over to Trotskyism and became an important leader of the YPSL's Trotskyist "Appeal Tendency" during 1936 and 1937. He was elected the organization's national secretary, its top post, at its September 1937 convention, which renounced the social democratic Second International in favor of a new Trotskyist Fourth International. The great majority of the YPSL supported this position and either left or was expelled by the Socialist Party in the fall of that year. Along with his YPSL activity, Draper took part in the founding of the Socialist Workers Party in 1937-1938.

As debates erupted within the newly formed SWP, Draper aligned with those who objected to the internal regime of that party and were developing an analysis of the Soviet Union under Joseph Stalin as a new form of society, neither socialist nor capitalist, in which a new class, the state bureaucracy, held social and state power. In 1940 this faction, led by Max Shachtman, James Burnham, and Martin Abern, split from the SWP to form the Workers Party. Draper joined them in founding the new organization. During the war, he and his wife Anne Draper, the former Anne Kracik, lived in Los Angeles where they were active among shipyard workers and in anti-fascist and anti-racist campaigns. Returning to New York in the mid-1940s, Draper became a major writer and functionary for the Workers Party. He would often write and edit almost the entire contents of issues of the group's paper, Labor Action.

By 1948 the Workers Party came to believe that the prospects for revolution were receding and that it must adopt a more realistic strategy given the diminished prospects. Therefore it changed its name to the Independent Socialist League, an acknowledgement that its size and capacities did not warrant the name "party." With a shrinking membership (although its youth work was buoyant) the ISL leadership around Shachtman decided that the time had come to join forces with the Socialist Party of America and in 1958 fused into it. Although Draper personally opposed this decision, he submitted to the majority. He regretted the rightward tendency of the organization, however, and in 1962, Draper ... by then resident in Berkeley, California, as a microfilm acquisitions librarian ... broke with the Socialist Party to form the Independent Socialist Club (ISC), which had a heavy youth composition and later in that decade became the International Socialists.

In 1964 Draper was heavily involved in the Free Speech Movement, an important precursor of that decade's New Left, on the University of California, Berkeley campus. He was a mentor to FSM leader Mario Savio and others. In the introduction to Draper's Berkeley: The New Student Revolt (1965), Mario Savio acknowledges Draper's encouragement and friendship, and cites the influence of Draper's pamphlet The Mind of Clark Kerr (October 1964) on the development of the Free Speech Movement.

In 1968 ISC became the International Socialists as it expanded nationally. In 1971 Draper quit, arguing that the group had become a sect. From then onwards he worked as an independent radical scholar, producing a stream of scholarly works on Marxism and the workers' movement. His scholarship on Marx was hailed by Robert Heilbroner in The New York Review of Books as "extraordinarily stimulating" and "written in a fresh, open style which comes as a welcome relief after the turgidities of so much Marxist writing."

Hal Draper died of pneumonia in 1990 at his home in Berkeley, California.

Draper's most enduring legacy is likely to be his five-volume magnum opus Karl Marx's Theory of Revolution, a seminal re-evaluation of the whole of Marx's political theory, based on an exhaustive survey of the writings of both Marx and Engels. He saw their political perspective as summarized by the phrase "socialism from below," which he had introduced in his pamphlet The Two Souls of Socialism.

Draper was also the editor of a three volume Marx-Engels Cyclopedia, a reference work detailing the day-to-day activities and writings of the two founders of modern socialism.

Outside his overtly political writings, Draper's most outstanding work is arguably the short story Ms Fnd in a Lbry, a satire of the information age, written in 1961.

In 1982, Draper also published an English translation of the complete works of the 19th Century German poet Heinrich Heine, the fruit of three decades of work conducted alongside his better-known political activity.

During his life, Hal Draper was a member of the following organizations:

• Young People's Socialist League
• Socialist Workers Party
• Independent Socialist League
• Socialist Party of America
• Free Speech Movement
• Independent Socialist Club

He was also a member of the editorial board of this journal:

• New Politics