Constantine was reputedly the son and successor of King Riderch Hael

Riderch I of Alt Clut

Riderch I , commonly known as Riderch or Rhydderch Hael , was a ruler of Alt Clut and the greater region later known as Strathclyde, a Brittonic kingdom that existed on the valley of the River Clyde in Scotland during the British Sub-Roman period...

 of Alt Clut

Kingdom of Strathclyde

Strathclyde , originally Brythonic Ystrad Clud, was one of the early medieval kingdoms of the celtic people called the Britons in the Hen Ogledd, the Brythonic-speaking parts of what is now southern Scotland and northern England. The kingdom developed during the post-Roman period...

, the Brittonic

Britons (historical)

The Britons were the Celtic people culturally dominating Great Britain from the Iron Age through the Early Middle Ages. They spoke the Insular Celtic language known as British or Brythonic...

 kingdom later known as Strathclyde (modern Dumbarton Rock). He appears only in the Life of St. Kentigern

Saint Mungo

Saint Mungo is the commonly used name for Saint Kentigern . He was the late 6th century apostle of the Brythonic Kingdom of Strathclyde in modern Scotland, and patron saint and founder of the city of Glasgow.-Name:In Wales and England, this saint is known by his birth and baptismal name Kentigern...

by Jocelyn of Furness

Jocelyn of Furness

Jocelyn of Furness was a Cistercian hagiographer, known for his Lives of Saint Waltheof, Saint Patrick, Saint Kentigern and Saint Helena....

, which regards him as a cleric, thus connecting him with the several obscure saints named Constantine

Constantine (British saint)

Saint Constantine is the name of one or many British or Pictish saints.- Identification :South-west BritainA Saint Constantine is revered in Devon and Cornwall. Based purely on similarity of a common name, some have identified him with the monarch Constantine of Dumnonia, despite the latter's...

 venerated throughout Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

.

According to Jocelyn, Constantine was the son of Riderch and his queen Languoreth. He succeeded his illustrious father upon his death, but later stepped down to become a clergyman. However, no other sources mention a son of Riderch named Constantine. He is absent from the pedigrees of Northern British kings in the Harleian genealogies

Harleian genealogies

The Harleian genealogies are a collection of Old Welsh genealogies preserved in British Library, Harleian MS 3859. Part of the Harleian Collection, the manuscript, which also contains the Annales Cambriae and a version of the Historia Brittonum, has been dated to c. 1100, although a date of c.1200...

 and the Bonedd Gwŷr y Gogledd

Bonedd Gwŷr y Gogledd

Bonedd Gwŷr y Gogledd is a brief Middle Welsh tract which claims to give the pedigrees of twenty 6th-century rulers of the Hen Ogledd , the Brythonic-speaking parts of southern Scotland and northern England. It is attested in a number of manuscripts, the earliest being NLW, Peniarth MS 45, which...

(This is the Descent of the Men of the North). A Saint Constantine was venerated in the area around Glasgow

Glasgow

Glasgow is the largest city in Scotland and third most populous in the United Kingdom. The city is situated on the River Clyde in the country's west central lowlands...

, the setting of much of Jocelyn's narrative; the early church in the nearby burgh of Govan

Govan

Govan is a district and former burgh now part of southwest City of Glasgow, Scotland. It is situated west of Glasgow city centre, on the south bank of the River Clyde, opposite the mouth of the River Kelvin and the district of Partick....

 was dedicated to him. However, by the 12th century Saint Constantine's biography was obscure, so it is likely that King Constantine was a literary invention created to provide a narrative for the shadowy early figure.

The compilation of hagiographies in the Orthodox Church known as the "Great Synaxaristes

Synaxarium

Synaxarion, Synexarion, pl. Synaxaria —Latin: Synaxarium, Synexarium—the name given in the Eastern Orthodox, Oriental Orthodox and Eastern Catholic Churches to a compilation of hagiographies corresponding roughly to the martyrology of the Roman Church.There are two kinds of synaxaria:*Simple...

" includes Saint Constantine of Strathclyde, giving his feast day as March 11. It also states that he was guided to Christianity by Saint Columba

Columba

Saint Columba —also known as Colum Cille , Colm Cille , Calum Cille and Kolban or Kolbjørn —was a Gaelic Irish missionary monk who propagated Christianity among the Picts during the Early Medieval Period...

, became a missionary of the faith in England and Ireland, and died around 640.