Archaeological material from the Bronze Age and earlier has been recovered and excavations have shown that there was unsustained late-Neolithic and Bronze Age human activity in the vicinity.
The earliest known permanent settlement dates from the Iron Age and began where a track running northwards from Bourne crossed the River Slea.
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.
The largest of its kind in Europe, the deposit has led archaeologists to consider that the site in Old Sleaford was probably one of the largest Corieltauvian settlements during this period and may have been a tribal centre.

During the Roman occupation of Britain (AD 43–409), the settlement was "extensive and of considerable importance."
Its location along the fen-edge may have made it economically and administratively significant as a centre for managers and owners of fenland estates.
There is evidence to suggest that a road connected Old Sleaford to Heckington (about 4 ¹⁄₂ mi or 7.2 km east), where Roman tile kilns have been uncovered and may imply the presence of a market at Sleaford.
When the first roads were constructed by the Romans, Sleaford was bypassed due to it being "less conveniently located" and more "geared to native needs".
A smaller road, Mareham Lane, which the Romans renewed, ran through Old Sleaford, and southwards along the fen edge, towards Bourne.
Where it passed through Old Sleaford, excavations have revealed a large stone-built domestic residence, associated farm buildings, corn-driers, ovens and field systems, all from the Roman period, and a number of burials.
Other Roman remains, including a burial, have been excavated in the town.
Archaeological material from the Bronze Age and earlier has been recovered and excavations have shown that there was unsustained late-Neolithic and Bronze Age human activity in the vicinity. The earliest known permanent settlement dates from the Iron Age and began where a track running northwards from Bourne crossed the River Slea. 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. The largest of its kind in Europe, the deposit has led archaeologists to consider that the site in Old Sleaford was probably one of the largest Corieltauvian settlements during this period and may have been a tribal centre.
During the Roman occupation of Britain (AD 43–409), the settlement was "extensive and of considerable importance." Its location along the fen-edge may have made it economically and administratively significant as a centre for managers and owners of fenland estates. There is evidence to suggest that a road connected Old Sleaford to Heckington (about 4 ¹⁄₂ mi or 7.2 km east), where Roman tile kilns have been uncovered and may imply the presence of a market at Sleaford. When the first roads were constructed by the Romans, Sleaford was bypassed due to it being "less conveniently located" and more "geared to native needs". A smaller road, Mareham Lane, which the Romans renewed, ran through Old Sleaford, and southwards along the fen edge, towards Bourne. Where it passed through Old Sleaford, excavations have revealed a large stone-built domestic residence, associated farm buildings, corn-driers, ovens and field systems, all from the Roman period, and a number of burials. Other Roman remains, including a burial, have been excavated in the town.
Sleaford