 Leśniewski married in 1913: his wife Zofia Prewysz-Kwinto came of a landed family from Kimborciszki (Kimbartiškė) in rural Lithuania. They had no children. Before World War I Leśniewski seems to have had the means to travel around to different German cities to study, and to spend time after his doctorate in Paris, San Remo and St. Petersburg.