Born at Haselau, near Uetersen, Holstein, Lehmann studied medicine in Copenhagen and Göttingen, obtained a doctorate in medicine in 1813 and a doctorate in philosophy from the University of Jena in 1814.

He spent the rest of his life as professor of physics and natural sciences, and head librarian, at the Gymnasium Academicum in Hamburg.

A prolific monographist of apparently quarrelsome character, he was a member of 26 learned societies and the founder of the Hamburg Botanical Garden (Botanischer Garten Hamburg, now the Alter Botanischer Garten Hamburg).

Some of Lehmann's later illustrations were executed by the German entomologist Johann Wilhelm Meigen.

Botanical specimens collected by Lehmann are cared for at institutions including the National Herbarium of Victoria (MEL), Royal Botanic Gardens Victoria.