He was born Tadeu Hâjdeu in Cristineștii Hotinului (now Kerstentsi in Chernivtsi Oblast, Ukraine), northern Bessarabia, at the time part of Imperial Russia.

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.

While the completed parts of the work do aim to be exhaustive, and are remarkably detailed, many of its entries reflect more of Hasdeu's own vision than historical facts (in one famous entry, he claims to be able to trace Basarab I's ancestry in a direct line to the Dacian rulers, with Dacia as a developed state that would have had, at times, dominated the Roman Empire—to the point where the single ruling family would have given Rome a large number of emperors).

Although he had been a staunch defender of the deposed Alexandru Ioan Cuza, he eventually backed the movement against him (led by Mihail Kogălniceanu), and was not opposed to the new Domnitor (future King of Romania) Carol I.

[citation needed] However, Hasdeu was a Liberal Party activist (he was elected to Parliament on its list for two non-consecutive terms), and close to its most radical, republican wing—the one led by C. A. Rosetti.

As the republican experiment coincided with worsened relations between Prime Minister Ion Brătianu and Carol, all Liberal Party members became suspect of involvement.

His works include two dramas, Răsvan și Vidra (romanticizing the actions of an obscure 1590s Moldavian-Romani Voivode, Ștefan Răzvan), and Domnița Ruxandra.