Algirdas was one of the seven sons of Grand Duke Gediminas. Before his death in 1341, Gediminas divided his domain, leaving his youngest son Jaunutis in possession of the capital, Vilnius. With the aid of his brother, Kęstutis, Algirdas drove out the incompetent Jaunutis and declared himself Grand Duke in 1345. He devoted the next thirty-two years to the development and expansion of the Grand Duchy of Lithuania.

After becoming the ruler of Lithuania, Algirdas was titled the King of Lithuania (Latin: rex Letwinorum) in the Livonian Chronicles instead of the Ruthenian terms knyaz (English: prince, duke) or velikiy knyaz (grand prince).

Two factors are thought to have contributed to this result: the political sagacity of Algirdas and the devotion of Kęstutis. The division of their dominions is illustrated by the fact that Algirdas appears almost exclusively in East Slavic sources, while Western chronicles primarily describe Kęstutis. Lithuania was surrounded by enemies. The Teutonic Order in the northwest and the Golden Horde in the southeast sought Lithuanian territory, while Poland to the west and Moscow principality to the east were generally hostile competitors.