Jacobi was born in Minneapolis, Minnesota in 1904 and lived there throughout his life. He was a lifelong bachelor. He was a voracious reader, gulping down at an early age quantities of Jules Verne, Edgar Allan Poe, H.G. Wells as well as the Frank Merriwell and Tom Swift boys' adventure yarns. Jacobi was always a writer; at his junior high school he earned good pocket-money concocting his own 'dime novels' (short story booklets) and selling them to fellow students as 10 cents-a-piece.