Praz was the son of Luciano Praz (died 1900), a bank clerk, and his wife, the former Giulia Testa di Marsciano (died 1931), daughter of Count Alcibiade Testa di Marsciano. His stepfather was Carlo Targioni (died 1954), a physician, whom his mother married in 1912. He studied at the
University of Bologna (1914–15), received a law degree from the
University of Rome (1918), and received a doctorate in literature from the
University of Florence (1920). Praz married, on 17 March 1934 (separated 1942, divorced 1947), Vivyan Leonora Eyles (1909–1984), an English-literature lecturer at the
University of Liverpool whom Praz met during his time there as a special lecturer in Italian studies. She was a daughter of the English novelist and feminist writer
Margaret Leonora Eyles (1889–1960), who addressed to her in 1941 an autobiographical work entitled
For My Enemy Daughter. She remarried in 1948, as her second husband, art historian Wolfgang Fritz Volbach. Praz and Eyles had one child, a daughter, Lucia Praz (born 1938). Praz's only other known romantic attachment was to an Anglo-Italian woman named Perla Cacciguerra, whom he met in 1953 and called "Diamante" in the book
The House of Life. Praz's residence in
Palazzo Primoli in Rome has been turned into the Museo Mario Praz. In
Orhan Pamuk's novel
The Museum of Innocence it is mentioned as "the most magnificent writer's museum I had seen". ==Life and writings==