After completing the Abitur in 1983, Lehmann apprenticed in a cabinetmaker's workshop — developing a rigorous understanding of materials, craft, and spatial thinking that still informs his work today. Followed by a subsequent internship at an architecture office.

He holds three graduate degrees: A master's degree from Mainz (1988) was followed by the AA Diploma from the Architectural Association in London (1991), completed on a DAAD scholarship. The AA's emphasis on advanced theory, distinctive avant-garde and experimental design left a lasting mark — shaping his commitment to urbanism as both an intellectual and practical discipline. The 1980s were an extremely interesting period at the AA School, influenced by the conceptual thinking of Team 10, Metabolism, and Archigram.

He later earned a doctorate in Architecture and Urbanism from the Technical University of Berlin (Dr.-Ing., 2003).

Before founding his own practice, he worked with two of the 20th century's most consequential architects: Pritzker laureate James Stirling in London, and Pritzker laureate Arata Isozaki in Tokyo. Both experiences sharpened his international perspective and instilled a standard of rigour.