Osten-Sacken was born in Reval, in present day Estonia, into the family of Baron Wilhelm-Ferdinand von der Osten-Sacken, who prior to his death in 1754, was a Captain-adjutant of Count Munnich. Baron Osten-Sacken was only two when his father died and spent his childhood in near-poverty. He entered the Kaporsky Musketry regiment as a sub-ensign on October 18, 1766.