Cyril (Russian Кирилл) was born in 1337 at Moscow to the noble Muscovite Cosmas Veliaminov family (orphaned in his youth). He entered the monastic life at the Simonov Monastery in Moscow — under the personal spiritual direction of St. Sergius of Radonezh (1314–1392, father of Russian monasticism).
Cyril served as abbot of the Simonov Monastery in Moscow from 1390 to 1394. In 1397, at the age of 60, Cyril undertook the vocation of founding a new monastery in the remote northern Russian forests — the pattern of the Sergian second-generation disciple-monks establishing the monastic-hermit foundations in the remote northern Russian territories.
Cyril and his companion St. Ferapont travelled north to the White Lake region (modern Vologda Oblast) and founded the Kirillo-Belozersky Monastery on Lake Siverskoye in 1397 — the principal Russian northern monastic centre.
The Kirillo-Belozersky Monastery — in continuous monastic use since 1397 — became one of the principal Russian northern monastic centres, producing multiple northern Russian monastic foundations (the 'Russian Northern Thebaid' tradition) and the famous Kirillo-Belozersky manuscript library (one of the principal medieval Russian manuscript collections, now preserved at the Russian National Library in Saint Petersburg).
Cyril served as abbot of Kirillo-Belozersky Monastery until his death on June 9, 1427.