There is little evidence of continuous settlement between the late Roman and Anglo-Saxon periods but the Saxons did establish themselves eventually.
South of the modern town, a 6th–7th century cemetery has been uncovered containing an estimated 600 burials, many showing signs of pagan burial rites.
The now ruined Church of St Giles/All Saints at Old Sleaford has been discovered and excavations of the market place in 1979 uncovered Anglo-Saxon remains from the 8th–9th centuries, indicating some form of enclosure with domestic features.
The earliest documentary reference to Sleaford is in a charter from the 9th century, when it was owned by Medehamstede Abbey in Peterborough, a Mercian royal foundation.
There is little evidence of estate structure until the late Saxon period, but there may have been a market and court in pre-Conquest Sleaford and it was probably an economic and jurisdictional centre for surrounding settlements.
The Slea played an important role in the town's economy: it never ran dry nor froze, and by the 11th century a dozen watermills lined its banks.
The mills and others in nearby Quarrington and the lost hamlet of Millsthorpe, constituted the "most important mill cluster in Lincolnshire".

Later in the Middle Ages, the Romano-British settlement became known as "Old Sleaford", while "New Sleaford" was used to describe the settlement centred on St Denys' Church and the market place.
The Domesday Book (1086) has two entries under Eslaforde (Sleaford) recording land held by Ramsey Abbey and the Bishop of Lincoln.
The location of the manors recorded in Domesday is not made clear in the text.
One theory endorsed by Maurice Beresford is that they focused on the settlement at Old Sleaford because of evidence that New Sleaford was planted in the 12th century by the bishop to increase his income, which was associated with the construction of Sleaford Castle between 1123 and 1139.
Beresford's theory has been criticised by the local historians Christine Mahany and David Roffe who have reinterpreted the Domesday material and argued that in 1086 the Bishop's manor included the church and associated settlement which became "New" Sleaford.

The right to hold a fair on the feast day of St Denis was granted by a charter of King Stephen to Alexander, Bishop of Lincoln, in 1136–40.
Between 1154 and 1165, Henry II granted the bishop of Lincoln the right to hold a market at Sleaford; bishop Oliver Sutton argued in 1281 that his right to hold a market and fair had existed since time immemorial.
In 1329, Edward III confirmed the market and, in 1401, Henry IV granted the bishop fairs on the feast days of St Denis and St Peter's Chains.
A survey of 1258 is the first to mention burgage tenure; tenants in the nearby hamlet of Holdingham held tofts with other land, while those in New Sleaford held only tofts, indicating that demesne farming centred on the hamlet.
The town later had at least two guilds comparable to those found in developed towns.
However, there was no formal charter outlining the town's freedoms; it was not a centre of trade and tight control by the bishops meant the economy was primarily geared to serve them.
Hence, it retained a strong tradition of demesne farming well into the 14th century.
As the economic initiative fell more to the burgesses and middlemen who formed connections with nearby towns, such as Boston, evidence suggests that Sleaford developed a locally important role in the wool trade.
In the Lay Subsidy of 1334, New Sleaford was the wealthiest settlement in the Flaxwell wapentake, with a value of £16 0s.
8d.¹/₄d.
Meanwhile, Old Sleaford, an "insignificant" place since the end of the Roman period, declined and may have been deserted by the 16th century.
There is little evidence of continuous settlement between the late Roman and Anglo-Saxon periods but the Saxons did establish themselves eventually. South of the modern town, a 6th–7th century cemetery has been uncovered containing an estimated 600 burials, many showing signs of pagan burial rites. The now ruined Church of St Giles/All Saints at Old Sleaford has been discovered and excavations of the market place in 1979 uncovered Anglo-Saxon remains from the 8th–9th centuries, indicating some form of enclosure with domestic features. The earliest documentary reference to Sleaford is in a charter from the 9th century, when it was owned by Medehamstede Abbey in Peterborough, a Mercian royal foundation. There is little evidence of estate structure until the late Saxon period, but there may have been a market and court in pre-Conquest Sleaford and it was probably an economic and jurisdictional centre for surrounding settlements. The Slea played an important role in the town's economy: it never ran dry nor froze, and by the 11th century a dozen watermills lined its banks. The mills and others in nearby Quarrington and the lost hamlet of Millsthorpe, constituted the "most important mill cluster in Lincolnshire".
Later in the Middle Ages, the Romano-British settlement became known as "Old Sleaford", while "New Sleaford" was used to describe the settlement centred on St Denys' Church and the market place. The Domesday Book (1086) has two entries under Eslaforde (Sleaford) recording land held by Ramsey Abbey and the Bishop of Lincoln. The location of the manors recorded in Domesday is not made clear in the text. One theory endorsed by Maurice Beresford is that they focused on the settlement at Old Sleaford because of evidence that New Sleaford was planted in the 12th century by the bishop to increase his income, which was associated with the construction of Sleaford Castle between 1123 and 1139. Beresford's theory has been criticised by the local historians Christine Mahany and David Roffe who have reinterpreted the Domesday material and argued that in 1086 the Bishop's manor included the church and associated settlement which became "New" Sleaford.
The right to hold a fair on the feast day of St Denis was granted by a charter of King Stephen to Alexander, Bishop of Lincoln, in 1136–40. Between 1154 and 1165, Henry II granted the bishop of Lincoln the right to hold a market at Sleaford; bishop Oliver Sutton argued in 1281 that his right to hold a market and fair had existed since time immemorial. In 1329, Edward III confirmed the market and, in 1401, Henry IV granted the bishop fairs on the feast days of St Denis and St Peter's Chains. A survey of 1258 is the first to mention burgage tenure; tenants in the nearby hamlet of Holdingham held tofts with other land, while those in New Sleaford held only tofts, indicating that demesne farming centred on the hamlet. The town later had at least two guilds comparable to those found in developed towns. However, there was no formal charter outlining the town's freedoms; it was not a centre of trade and tight control by the bishops meant the economy was primarily geared to serve them. Hence, it retained a strong tradition of demesne farming well into the 14th century. As the economic initiative fell more to the burgesses and middlemen who formed connections with nearby towns, such as Boston, evidence suggests that Sleaford developed a locally important role in the wool trade. In the Lay Subsidy of 1334, New Sleaford was the wealthiest settlement in the Flaxwell wapentake, with a value of £16 0s. 8d.¹/₄d. Meanwhile, Old Sleaford, an "insignificant" place since the end of the Roman period, declined and may have been deserted by the 16th century.
Sleaford