Mustawfi was born in 1281 in the town of Qazvin, located in Persian Iraq ("Irāq-i Ajam"), a region corresponding to the western part of Iran. His family was descended from Arabs that had occupied the governorship of the town in the 9th and 10th-centuries, later to serve as "mustawfis" (high-ranking financial accountants) at the advent of the Ghaznavids. Mustawfi's great-grandfather Amin al-Din Nasr had served as the "mustawfi" of Iraq, which had since then been the moniker of the family. Amin al-Din Nasr, during his retirement, was killed in 1220 by raiding Mongols after the sack of Qazvin, during the Mongol invasion of Iran.