Lorentowicz was born in Pabianice, in present-day central Poland, in 1868, five years after the opening of the Polish 1863 Uprising that was eventually suppressed by the Russian Imperial Army. His parents had moved to present-day central Poland from the old eastern "Kresy" borderlands, escaping poverty and oppression. Jan obtained his secondary-school diploma at Płock by way of home schooling and soon went to France, where he lived until 1903 and studied at the Sorbonne.