Lehnert was born in Gross Aupa in Bohemia, Austria-Hungary (now Velká Úpa, part of Pec pod Sněžkou in the Czech Republic).

[1] He first travelled to Tunis in 1904, and in 1904 he again visited with his friend, and subsequent business partner, Ernst Heinrich Landrock.

The pair established a photographic studio in Tunis and worked closely for more than 20 years.

They later established studios in successively, Munich, Leipzig and Cairo, publishing the works as by "Lehnert & Landrock".

[2] Lehnert spent the last part of his life at Redeyef, Gafza Oasis, Tunisia, where he died.