He was the only child of Ernst Moritz Mendelssohn and Elizabeth Ruprecht. Through his grandfather he was a great-great-grandson of Saul Mendelssohn, the younger brother of philosopher Moses Mendelssohn. This places him amongst the Mendelssohn family. Francis Simon and Heinrich Mendelssohn were his cousins. He married Jutte Zarniko, the sister of Barbara Zarniko, one of Franz Simon's students in 1932. When James Crowther married Franziska Zarniko in 1934, he became a brother-in-law of Mendelssohn.