Lachnit was born in the small town of Gittersee; his family moved to Dresden in 1906. He studied at the Kunstgewerbeschule Dresden under Richard Guhr, and later at the Dresden Academy of Fine Arts, where he was acquainted with and influenced by Otto Dix, Conrad Felixmüller, and Otto Griebel. He joined the Communist Party of Germany in 1924 and was active in producing various forms of Agitprop throughout the 1920s. He co-founded the "Neue Gruppe" with Hans Grundig, Otto Griebel, and Fritz Skade; successful exhibitions in Paris, Düsseldorf, Amsterdam, and Dresden followed.
After the Nazis seized power in 1933, Lachnit's work was declared "degenerate" and confiscated by authorities. During this period he was not allowed to make art and worked as an exhibition designer. Much of his confiscated work was destroyed during the February 1945 firebombing of Dresden. His 1923 watercolours Man and Woman in the Window and "Girl at Table" were found in the 2012 Nazi loot discovery.
Lachnit continued to paint after the end of World War II. In 1947 he was appointed professor at the Hochschule für Bildene Künste Dresden. Among his more important students were Jürgen Böttcher, Manfred Böttcher, and Harald Metzkes.
- Joachim Uhlitzsch: Wilhelm Lachnit. Seemann, Leipzig 1968.
- Fritz Löffler (1982), "Lachnit, Wilhelm", Neue Deutsche Biographie (NDB) (in German), 13, Berlin: Duncker & Humblot, pp. 378–379
- Friedegund Weidemann: Der Maler und Graphiker Wilhelm Lachnit: Studie zu seinem Menschenbild. Diss, Humboldt-Univ., Berlin 1983.
- Hans Joachim Neidhardt: Dresden, wie es Maler sahen. Edition Leipzig, Leipzig 1983.
- Wilhelm Lachnit. Gemälde, Graphik, Zeichnungen. Akademie der Künste der DDR, Kathleen Krenzlin, Berlin 1990. ISBN 3-86050-010-4
- Ingrid Wenzkat (ed.): Dresden - Vision einer Stadt. Hellerau-Verlag Dresden, Dresden 1995.
- Gabriele Werner: Wilhelm Lachnit, Gemälde 1899-1962; Ausstellung vom 12. Februar bis 30. April 2000 Staatliche Kunstsammlungen Dresden, Gemäldegalerie Neue Meister, Albertinum Brühlsche Terrasse. Staatliche Kunstsammlungen, Dresden 1999.
- Wulf Kirsten und Hans-Peter Lühr (ed.): Künstler in Dresden im 20. Jahrhundert. Literarische Porträts. Verlag der Kunst Dresden, Dresden 2005.