Seyidagha Onullahi was born in the city of Astara, Southern Azerbaijan, on 26 August 1925. He got his primary education in Farsi. In 1945-1946, he participated in the Southern Azerbaijan national liberation movement. He was forced to move to Soviet Azerbaijan after the fall of the national government. In 1952, he graduated from the State Pedagogical Institute named after H.Zardabi, Faculty of History in Ganja. In 1958, he embarked on postgraduate studies at Baku State University, Faculty of Oriental Studies. In 1963, he defended a PhD thesis, and achieved a ScD in 1983 and professorship in 1985. In 1962, Seyidagha Onullahi joined the Institute of History, Academy of Sciences, Azerbaijan SSR, as a junior research fellow, and was then promoted to a senior and then a leading research fellow. From 2001 to 2004, he headed the Department of Ancient and Medieval Ages History of Azerbaijan, Baku State University. In 1945-1946, he was awarded “21 Azeris” Medal for his participation in the Southern Azerbaijan national liberation movement. His research interests laid in major problems of Medieval period of Azerbaijan`s history. He published a number of monumental monographs and over 100 papers.