Against his own wishes, his father sent him to the Hasidic rabbi of Ger to study. He was ordained as a rabbi by virtue of his brilliant speech on the occasion of his bar mitzvah.
His first poems, published in Polish literary periodicals when he was sixteen years old, still had strong Jewish ties. He left Cracow for Warsaw at age eighteen, making a gradual transition from poetry to literary criticism, philosophy, and finally Marxism and his ultimate rejection of Judaism.
In 1927 he joined the banned Polish Communist party, soon becoming the chief editor of the clandestine Communist press.
In 1932 he was expelled from the Communist party, ostensibly for “exaggerating the threat of Nazism and spreading panic in the Communist ranks.” This was due to his having formed, upon his return from the Soviet Union, an anti-Stalinist splinter group within the Polish Communist party. His publication of an article entitled “The Danger of Barbarism over Europe" led to his expulsion.
He traveled to the Soviet Union in 1934, rejecting a number of academic positions that had been offered to him.
In 1939 Deutscher became the London correspondent for a Polish Jewish newspaper. When the war broke out he was forced to find a way of earning a living in England. He devoted himself to learning English with the same zeal he had applied as a child to his Talmudic studies. His first article in English appeared in The Economist; this marked the beginning of his career in English-language journalism.'He joined the Polish army in Scotland in 1940, but during a large part of his service he was considered a “subversive element” due to his protests against anti-Semitism in the army.
In 1942 he became The Economist’s expert on Soviet affairs and its chief European correspondent. At the same time he wrote for The Observer under the pseudonym “Peregrine.”
Deutscher’s first book on Soviet affairs, "Stalin: A Political Biography" (1949), was widely translated. It was followed by a trilogy on Trotsky, " The Prophet Armed"( 1954), "The Prophet Unarmed" (1959) and "The Prophet Outcast" (1963). Much of the material for his trilogy was gleaned from the closed section of Harvard University’s Trotsky archives, which is to remain closed until the end of the twentieth century, and to which he obtained access by special permission of Trotsky’s widow.
Deutscher died in Rome before he was able to fulfill his ambition to write a biography of Lenin.
He called himself a "non-Jewish Jew," and his autobiography, published posthumously, bears that title.
He was opposed to any form of nationalism, viewing it as an historically retrogressive movement. His attitude toward Zionism was no different, although he conceded that "if, instead of arguing against Zionism in the 1920s and 1930s, I had urged European Jews to go to Palestine, I might have helped to save some of the lives that were later extinguished in Hitler’s gas chambers."
He was married to Tamara Deutscher.