The earliest records of the place-name Sleaford are found in a charter of 852 as Slioford and in the Anglo-Saxon Chronicle as Sliowaford.
In the Domesday Book (1086), it is recorded as Eslaforde and in the early 13th century as Sliforde.
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, 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.
Granted the right to hold a market in the mid-12th century, New Sleaford developed into a market town and became locally important in the wool trade, while Old Sleaford declined.
From the 16th century, the landowners were the Carre family, who operated tight control over the town, and it grew little in the early modern period.
With a population of 17,671 at the 2011 Census, the town is the largest settlement in the North Kesteven district.
Bypassed by the A17 and the A15, it is connected to Lincoln, Newark, Peterborough and King's Lynn.
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 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.In the medieval period, records differentiate between Old and New Sleaford, the latter emerging in the areas around the present day market place and St Denys' Church.Sleaford Castle was constructed in the 12th century for the Bishops of Lincoln, who owned the manor.Sleaford was primarily an agricultural town until the 20th century, supporting a cattle market, with seed companies, such as Hubbard and Phillips, and Sharpes International Seeds, being established in the late 19th century.The arrival of the railway made the town favourable for malting.Although only sparse pottery evidence has been found for the middle Iron Age period, 4,290 pellet mould fragments, likely used for minting and dated to 50 BC–AD 50, have been uncovered south east of the modern town centre, south of a crossing of the River Slea and near Mareham Lane in Old Sleaford.