He married Gertrud Sabersky, a student of the artist Walter Leistikow, and his own portrait was painted by Lovis Corinth in 1899.
[4] After a hand injury meant that he could no longer perform surgery, he turned to writing about antiquity.
[7] Mainzer was a close friend of the Catholic priest Friedrich von Erxleben, who was a member of the Solf Circle of intellectuals involved in the resistance against Nazism.
[5] Mainzer and his family were helped to escape to England by the daughter of Wilhelm and Hanna Solf, the Countess So'oa'emalelagi "Lagi" von Ballestrem-Solf, who escorted them with their jewellery hidden in the lining of her clothes.
In May 2021, a portrait of Ferdinand Mainzer by Lovis Corinth was accepted for the nation in lieu of a UK inheritance tax bill.