Lehnert was born in Gross Aupa (now Velká Úpa), then in Bohemia and now in the Czechoslovakian Republic.
With Ernst Heinrich Landrock, he based a photographic company in, successively, Tunis, Munich, Leipzig and Cairo, publishing the works as by "Lehnert & Landrock". From the 1860s onwards photographs of people with different cultural values and sexual morality became popular for artistic and erotic reasons. These nude images often say more about the fantasies and culture of the photographers than about the portrayed cultures.
According to Pascal Baetens they border on racism and ethnocentrism.
Lehnert spent the last part of his life at Redeyef, Gafza Oasis, Tunisia, where he died.