He married Sophie Marie Katharina Seyler (1762–1833) in Hamburg in 1781. She was the daughter of famed Swiss-born theatre director Abel Seyler and stepdaughter of actress Friederike Sophie Seyler, and grew up with her uncle J.G.R. Andreae in Hanover. Her brother was banker Ludwig Erdwin Seyler, who became by marriage a member of the Berenberg-Gossler banking dynasty. Leisewitz was a distant relative of his wife on the Andreae side, and had been a frequent visitor in the Andreae home, with its large library, in his youth. He would later refer to J.G.R. Andreae as his uncle. His diaries and his letters to his wife have been published.