Sigel was born in Sinsheim, Baden (Germany). He attended the gymnasium in Bruchsal. He graduated from Karlsruhe Military Academy in 1843, and was commissioned as a lieutenant in the Baden Army. In 1848 he was a part of the Revolutions of 1848 in the German states. When the revolution failed he fled, eventually immigrating to New York City. There he worked at a number of different jobs. These include a teacher, musician, surveyor and the owner of a cigar store. In 1854 Sigel married Elsie Dulon. Together they had five children. In 1857 he moved to St Louis, Missouri where he was one of the founders of a monthly magazine. He was the superintendent at the German Institute and joined the Republican Party. He attracted German immigrants to the Republican party because of their strong anti-slavery sentiment.