Before the Norman conquest a small Saxon chapel dedicated to St Peter stood outside the east gate of Shrewsbury; it had been built by Siward, son of Ethelgar and a close relative of Edward the Confessor.
There was still a landowner, known as Siward the Fat, in Shropshire at Domesday, although he had owned many more estates in 1066.
He must have been the donor of the two estates the church is known from Domesday to have held in 1066: at Boreton near Condover and Lowe near Farley.
However, the Abbey had lost Lowe by 1087.

When Roger de Montgomery received Shropshire from William the Conqueror in 1071, he gave the church to one of his clerks, Odelerius of Orléans, the father of the historian Orderic Vitalis, who is the main source for the foundation of the Abbey and probably an eye-witness.
Orderic stresses his father's role in persuading Earl Roger to commit himself to building a monastery and stresses that Odelerius from the outset wanted it to be Benedictine.
The specific purpose was to benefit Earl Roger's soul.

Unde generositatem vestram, gloriose consul, fideliter admoneo ut, dum licet, in comitatu vestro (quem jure a patribus non consecutus es haereditario) monachile castrum contra Satanan construatur Deo; ubi pro anima vesita cucullati pugiles Behemoth conflictu resistent assiduo.

I therefore offer to you, most noble earl, my faithful advice, that while it is in your power, you call a stronghold for monks against Satan to be built for the service of God in the chief seat of your earldom, which is not yours by inheritance from your ancestors, in order that these cowled combatants may withstand the devil in continual conflict for the good of your soul.

On 25 February 1083 Earl Roger summoned his senior officials, including Warin, the Sheriff of Shropshire and Picot de Say, and publicly pledged himself to found a new Abbey, laying his gloves on the altar of St. Peter and granting the whole suburb outside the east gate for the construction.
Reginald and Frodo, two monks from the great Benedictine Abbey of Saint-Martin-de-Séez in Southern Normandy, formed the nucleus of the new community, and began to plan and build the monks' lodging, working with Ordelerius and Warin.
Domesday Book found the abbey under construction: "In the City of SHREWSBURY Earl Roger is building an Abbey and has given to it the monastery of St Peter where the parish (church?)
of the City was."
This suggests that Siward's foundation was already a monastery before Earl Roger began building but it is fairly certain there was no more than a wooden parish church.

When sufficiently complete (probably late in 1087), regular life began under the first abbot, Fulchred of Sées.
The Abbey of Saint-Martin-de-Sées was closely associated with Shrewsbury in the early years because Earl Roger was its founder and he and the House of Bellême, into which he had married, were also major benefactors there, as were Roger's knights.
A notification lodged at Sées in 1086 by Robert of Bellême, Roger's son, who was later to become 3rd Earl of Shrewsbury, shows that they used the monks as witnesses in their property deals and custodians of the documents.
About the time he recruited Abbot Fulchred or Foucher, Earl Roger made a huge grant of estates in England to Sées, for the soul of recently deceased William the Conqueror, as well as of Queen Matilda and of Roger himself and his dead wife Mabel de Bellême.
Some of these were estates he had originally intended for Mabel.
It seems that Sées Abbey for a time harboured plans to claim jurisdiction over Shrewsbury and it also contested some properties granted by Earl Roger; however, Shrewsbury became independent.

Once it was safely under the leadership of Fulchred, Ordelirius placed Benedict, one of his sons, in Shrewsbury Abbey as an oblate, with a gift of 200 silver livres.
Both Ordelirius himself and Earl Roger met their deaths as monks of the Abbey.
Before the Norman conquest a small Saxon chapel dedicated to St Peter stood outside the east gate of Shrewsbury; it had been built by Siward, son of Ethelgar and a close relative of Edward the Confessor. There was still a landowner, known as Siward the Fat, in Shropshire at Domesday, although he had owned many more estates in 1066. He must have been the donor of the two estates the church is known from Domesday to have held in 1066: at Boreton near Condover and Lowe near Farley. However, the Abbey had lost Lowe by 1087.
When Roger de Montgomery received Shropshire from William the Conqueror in 1071, he gave the church to one of his clerks, Odelerius of Orléans, the father of the historian Orderic Vitalis, who is the main source for the foundation of the Abbey and probably an eye-witness. Orderic stresses his father's role in persuading Earl Roger to commit himself to building a monastery and stresses that Odelerius from the outset wanted it to be Benedictine. The specific purpose was to benefit Earl Roger's soul. 
Unde generositatem vestram, gloriose consul, fideliter admoneo ut, dum licet, in comitatu vestro (quem jure a patribus non consecutus es haereditario) monachile castrum contra Satanan construatur Deo; ubi pro anima vesita cucullati pugiles Behemoth conflictu resistent assiduo.
I therefore offer to you, most noble earl, my faithful advice, that while it is in your power, you call a stronghold for monks against Satan to be built for the service of God in the chief seat of your earldom, which is not yours by inheritance from your ancestors, in order that these cowled combatants may withstand the devil in continual conflict for the good of your soul.
On 25 February 1083 Earl Roger summoned his senior officials, including Warin, the Sheriff of Shropshire and Picot de Say, and publicly pledged himself to found a new Abbey, laying his gloves on the altar of St. Peter and granting the whole suburb outside the east gate for the construction. Reginald and Frodo, two monks from the great Benedictine Abbey of Saint-Martin-de-Séez in Southern Normandy, formed the nucleus of the new community, and began to plan and build the monks' lodging, working with Ordelerius and Warin. Domesday Book found the abbey under construction: "In the City of SHREWSBURY Earl Roger is building an Abbey and has given to it the monastery of St Peter where the parish (church?) of the City was." This suggests that Siward's foundation was already a monastery before Earl Roger began building but it is fairly certain there was no more than a wooden parish church.
When sufficiently complete (probably late in 1087), regular life began under the first abbot, Fulchred of Sées. The Abbey of Saint-Martin-de-Sées was closely associated with Shrewsbury in the early years because Earl Roger was its founder and he and the House of Bellême, into which he had married, were also major benefactors there, as were Roger's knights. A notification lodged at Sées in 1086 by Robert of Bellême, Roger's son, who was later to become 3rd Earl of Shrewsbury, shows that they used the monks as witnesses in their property deals and custodians of the documents. About the time he recruited Abbot Fulchred or Foucher, Earl Roger made a huge grant of estates in England to Sées, for the soul of recently deceased William the Conqueror, as well as of Queen Matilda and of Roger himself and his dead wife Mabel de Bellême. Some of these were estates he had originally intended for Mabel. It seems that Sées Abbey for a time harboured plans to claim jurisdiction over Shrewsbury and it also contested some properties granted by Earl Roger; however, Shrewsbury became independent.
Once it was safely under the leadership of Fulchred, Ordelirius placed Benedict, one of his sons, in Shrewsbury Abbey as an oblate, with a gift of 200 silver livres. Both Ordelirius himself and Earl Roger met their deaths as monks of the Abbey.
Shrewsbury Abbey