Sibthorp House was located on the High Street just north of the present-day magistrates' court. Built during the 16th century, the single-storey facade was replaced in the Queen Anne Style in the early 18th century for John Sibthorpe, M.P. Sibthorpe had planned to add an upper storey but died in 1716 before the work could be carried out. His widow, Mary Sibthorp, stopped the building and soon afterwards bought Canwick Hall.
Her story was a sad one, in 1726 she lost her son Gervase, aged only 16 and the next year Mary's eldest son, John, aged 22, who, with their parents, were buried in the family vault of St Mark’s church.
The lower rooms were large well-proportioned apartments but the upper rooms were garrets opening into one another and quite out of keeping with the lower rooms of the house. The house incorporated a very early mansion of which we know nothing, probably dating from the 14th century. In the lower part of the back of the house were Gothic doorways and at right angles with it another part of the ancient building.
For many years it was the Inland Revenue tax office, the LNER locomotive control depot and later British Railways Eastern Region Offices.
In 1938 a Rhesus Monkey named Jacko travelling by rail from Liverpool to Grantham escaped from a crate and climbed onto the roof Sibthorp House, removed tiles and entered the roof space. He was eventually recaptured by using another monkey to entice him into a room into Sibthorp House.
Sibthorp House was demolished in 1962 and modern shops and offices were built on the site, also named Sibthorp House.