Alexius took monastic vows in Bogoyavlensky Monastery around 1313. In 1333 or so, he moved to Moscow at the request of the Grand Prince and Metropolitan Theognostus. In 1340, Alexius was appointed Metropolitan's deputy in Vladimir and 12 years later would become the Bishop of Vladimir. By the will of Semeon of Russia, Alexius was appointed adviser to Semeon's brothers - Ivan and Andrei. After visiting Constantinople, he was chosen to become the Metropolitan of Kiev and all Russia in 1354. When Dmitry Donskoy was young, Alexius was his spiritual tutor and a regent at the same time. He took the side of Dmitry Donskoy in his struggle against Tver (1366-1370).
On September 4, 1943 Alexius I together with a delegation of senior Russian Orthodox clerics met with Joseph Stalin in the Kremlin where a historic decision was made regarding the fate of the Church in the state ruled by militantly atheist Communist party.
On February 2, 1945 Alexius I was elected Patriarch of Moscow and all of Russia.
Alexius died from infarct at the age of 91 and was buried in Troitse-Sergiyeva Lavra in 1970.