Unruh was born in Koblenz, Germany. A general's son, he was an officer in the German army until 1912, when he left to pursue his writing career. Two of his earliest important works, the play Offiziere ("Officers"; 1911) and the poem Vor der Entscheidung ("Before the Decision"; 1914) established his anti-war beliefs and his belief that the social order must be based not on authority, but on the integrity and responsibility of the individual towards humanity. Unruh's works were anti-militaristic and called for world peace and brotherhood. Some of his more notable works include Der Opfergang ("Way of Sacrifice"), a powerful anti-war piece written during the siege of Verdun and published in 1919, Ein Geschlecht ("A Family"; 1916) and its sequel Platz (1920), and Heinrich von Andernach (1925).
Unruh was a staunch opponent of the Nazi Party and wrote several works warning of the consequences of Nazi dictatorship, including Bonaparte (1927), Berlin in Monte Carlo (1931), and Zero (1932). He left Germany for France in 1932, later immigrating to the United States. He finally returned to Germany in 1962 and died in the town of Dietz at age 85.