Schleiden was born at Hamburg and studied law at Heidelberg; he then returned to Hamburg to practice as a lawyer. However, he soon became fully occupied by his interest in botany and graduated in 1831 from the University of Jena, where he became professor of botany in 1839.

Instead of becoming involved in plant classification – the pursuit of most of his botanical contemporaries – Schleiden studied plant growth and structure under the microscope. This led to his Contributions to Phytogenesis (1838), which stated that the various structures of the plant are composed of cells or their derivatives. He thus formulated the cell theory for plants, which was layer eleborated and extended to animals by the German physiologist Theodor Schwann. Schleiden recognized the significance of the cell nucleus and sensed its importance in cell division, although he thought (wrongly) that new cells were produced by budding from its surface. He was one of the first German biologists to accept Darwinism.