 Otto Sinding was born in Kongsberg as a son of mine superintendent Matthias Wilhelm Sinding (1811–1860) and Cecilie Marie Mejdell (1817–86).He was the older brother of the sculptor Stephan Sinding and the composer Christian Sinding.He was a nephew of Nicolai Mejdell (1822–1899) and Thorvald Mejdell (1824–1908), and through the former a first cousin of Glør Thorvald Mejdell, who married Otto's sister Thora Cathrine Sinding.