Brun was born in to the family of a tobacco factory owner. In 1902 he was expelled from the pro-gymnasium for participating in the May Day demonstration. In 1903 he joined the Union of Socialist Youth and co-edited it magazine "Ruch". In February 1904 he was arrested for political activity, in May he was released due to lack of evidence. He was arrested again during a socialist demonstration at Grzybowski Square in Warsaw on November 13, 1904. He was detained in Pawiak and in the
Warsaw Citadel. In 1905 he a member of the
Social Democracy of the Kingdom of Poland and Lithuania (SDKPiL) and was active in Warsaw, Lublin and the Dąbrowa Basin. In 1906 he went to Paris, where he studied sociology at the
Sorbonne and was active in the Paris section of the SDKPiL. In 1908 he married the English painter Mary Houghton and in 1909 he studied photoengraving in London. In the autumn of 1912 he went to Kraków, where he married again to the SDKPiL activist Stefania Unszlicht, sister of
Józef Unszlicht. From the spring of 1913 to 1919 he lived in Bulgaria. In 1919 he returned to Poland and became a member of the newly founded
Communist Party (KPP). In 1923 he was elected a member of the Central Committee of the Communist Party of Poland. At the same time he became a member of the
Communist Party of Western Belorussia (KPZB) and was soon elected to its central committee. In 1924 he was arrested by the Polish authorities for anti-state activity and sentenced to eight years in prison. In 1926 he arrived in the Soviet Union as part of an exchange of political prisoners after which he became a correspondent for the
Soviet Telegraph Agency. From 1929 he lived in France and Belgium and continued to be active in the leading organs of the KPP and KPZB. He lived in Saratov, where he served as editor of the Polish editorial board of the Ukrainian radio station. He died on April 28, 1942, in Saratov after a long illness. == Commemoration ==