Kevin, a descendant of one of the ruling families in Leinster, studied as a boy under the care of three holy men, Eoghan, Lochan, and Eanna. During this time, he went to Glendalough. He was to return later, with a small group of monks to found a monastery where the 'two rivers form a confluence'. Kevin's writings discuss his fighting "knights" at Glendalough; scholars today believe this refers to his process of self-examination and his personal temptations. His fame as a holy man spread and he attracted numerous followers. He died in about 618, traditionally on 3 June. For six centuries afterwards, Glendalough flourished and the Irish Annals contain references to the deaths of abbots and raids on the settlement.
Around 1042, oak timber from Glendalough was used to build the second longest (30 m) Viking longship ever recorded. A modern replica of that ship was built in 2004 and is currently located in Roskilde, Denmark.
At the Synod of Rath Breasail in 1111, Glendalough was designated as one of the two dioceses of North Leinster.
The Book of Glendalough was written there about 1131.
St. Laurence O'Toole, born in 1128, became Abbot of Glendalough and was well known for his sanctity and hospitality. Even after his appointment as Archbishop of Dublin in 1162, he returned occasionally to Glendalough, to the solitude of St. Kevin's Bed. He died in Eu, in Normandy in 1180.
In 1176, the Annals of Tigernach report that Glendalough was 'plundered by the foreigners'.
In 1214, the dioceses of Glendalough and Dublin were united. From that time onwards, the cultural and ecclesiastical status of Glendalough diminished. The destruction of the settlement by English forces in 1398 left it a ruin but it continued as a church of local importance and a place of pilgrimage.
Glendalough features on the 1598 map "A Modern Depiction of Ireland, One of the British Isles" by Abraham Ortelius as "Glandalag".
Descriptions of Glendalough from the 18th and 19th centuries include references to occasions of "riotous assembly" on the feast of St. Kevin on 3 June.
The present remains in Glendalough tell only a small part of its story. The monastery in its heyday included workshops, areas for manuscript writing and copying, guest houses, an infirmary, farm buildings and dwellings for both the monks and a large lay population. The buildings which survive probably date from between the 10th and 12th centuries.