Janco was born in Bucharest, Romania. In 1910–14 he exhibited at the salons in Bucharest and moved among modernist artists and poets. In 1916, while studying architecture, he was among the founders of Dada in Zurich. There he participated in the famous evenings at Café Voltaire where he was in charge of the stage and costume design. In the 1920s he was much involved in the Dada movement. He had ties with the Paris branch, participating there in an international exhibition of abstract art, and was one of the founders of the art and literature journal Contimporanul. In 1940, following the rise of fascism in Romania, he immigrated with his family to Ereẓ Israel. In Israel, Janco participated in many important exhibitions including those of New Horizons and the Venice Biennale. In 1953 he established the Ein Hod Artists Village and founded the Department for Art Teachers at the Oranim College. In 1967 he was awarded the Israel Prize. In 1983 he was involved in the establishment of the Janco-Dada Museum in Ein Hod.

The art style of Janco moved between the figurative and the abstract. In Israel he had many subjects: soldiers and battle situations, transit camps and immigrant types, Arabs, landscapes, and the Holocaust.

The wounded soldier virtually became the symbol of his work (Wounded Soldier, 1949, Israel Museum, Jerusalem). These soldiers had complex meanings. In some of the paintings they looked as if crucified, or praying, or like the figures of Picasso's Guernica.

Janco dealt with the theme of immigration from every possible angle. He described the crowded ships, the refugees stealing into the country sheltered by the darkness of night, particular immigrant groups like the Yemenites, tents and figures in the transit camps. In all of these paintings the expressive style seemed to be a reflection of his own experience.

Janco's arrival in Israel after many years on the world stage represented a significant contribution to Israeli art. Within weeks of his arrival, a group of gifted Israeli painters gathered around him, seeing an opportunity through him of effecting a desired change in Israel's artistic life. Janco's activism helped fulfill their expectations.