He was born in Cristinestii Hotinului (now Kerstenzy, where is near Khotyn), northern Bessarabia, at the time part of Imperial Russia, into a family with noted Polish connections. After studying law at the University of Kharkiv, he fought as a Russian hussar in the Crimean War. In 1858 he settled in Iasi as a high school teacher and librarian. In 1865, Hasdeu published a monograph on Ioan Voda the Terrible, renaming him for the first time cel Viteaz—"the Brave". The portrayal of this violent, short rule as a glorious moment (and of Ioan himself as a reformer) drew criticism from the Junimea society, a conflict which was to follow Hasdeu for the rest of his life. Still, Hasdeu's version of Ioan's character and his anti-boyar actions were to be reclaimed as a founding myth by Communist Romania.
In 1863, Hasdeu again moved his residence, from Iasi to Bucharest; he began editing the satirical magazine Aghiuta, which ceased publication the following year.