Alexius, whose name at birth was Elephtherios, was a son of Fyodor (Theodore) Biakont, a boyar from Chernigov who settled in Moscow and founded the great Pleshcheev boyar family. He took monastic vows at the Epiphany Monastery of Moscow around 1313, at which time he was given the religious name of Alexius. In 1333 or so, he joined the household of Metropolitan Theognostus. In 1340, Alexius was appointed the Metropolitan's deputy in Vladimir and twelve years later was consecrated as Bishop of Vladimir.
By the will of Symeon the Proud, Alexius was appointed adviser to his brothers - Ivan and Andrew. After visiting Constantinople, he was chosen to become the Metropolitan of Moscow and all Russia in 1354. When Dmitrii Donskoi and Vladimir the Bold were young, Alexius was their spiritual tutor and served as regent at the same time. He took the side of Dmitrii Donskoi in his struggle against Tver and Nizhny Novgorod, where he once sent St. Sergius of Radonezh to suspend divine service in churches and monasteries, until the political strife was over.
In 1360s, Alexius founded the Andronikov, Chudov, and Alekseyevsky monasteries. He promoted Metropolitan Peter's canonization by the Russian Orthodox Church. Shortly before his death, Alexius fruitlessly tried to convince Sergius of Radonezh to become his successor.
Alexius was also an author of a number of sermons and epistles. He was glorified (canonized) by the Russian Orthodox Church in 1448 and has been revered as one of the patron saints of Moscow. His relics were venerated in the Chudov Monastery that he had founded.