Mossinson, son of Asher Mossinson and his wife Dvora, was born in Ottoman Palestine in 1917, in the moshav Ein Ganim, located near Petah Tikva; he grew up in Tel Aviv. Later on Mossinson studied in Beit Alfa and in the youth village of Ben Shemen. Afterward he moved to the Kibbutz Na'an, where he lived from 1938 to 1950. In 1943 Mossinson joined the Palmach. During that period Mossinson was arrested by the British and imprisoned in Latrun. In 1944 Mossinson published his first story in the newspaper Al HaMishmar. During the 1948 Arab–Israeli War Mossinson served as a cultural officer in the Givati unit.