He was born to Salomon Emanuel Unna (1780-1819), a furniture dealer and auctioneer, and his wife, Jacobine Jacobsen.
After finishing school, he worked in a trading office as a clerk, but wanted to be a painter.
He therefore returned to Copenhagen in 1830, where he was accepted as a student at the Royal Danish Academy of Fine Arts.
In 1850, he found employment as a drawing teacher, and began creating scenes of Swedish folk life.
His income was still insufficient to provide a decent living, so he took up photography; opening a studio in 1854.