It is likely that there has been a church on this site since Saxon times. The north wall of the nave is the oldest part of the church and may include portions of Anglo-Saxon work.
The building was extensively remodelled in the early 13th Century with the construction of the north aisle and the chancel; the three rather crude arches to the north aisle appear to have been cut through the original nave. On the south side there are two late 13th Century arches. The clerestory was added to the nave in the 15th Century. The roof was reconstructed in 1904/1905 when the old galleries at the west end were removed.
Redecoration of the church in 1995 confirmed that the plaster on the nave walls (apparently applied in haste at the time of the Reformation) conceals extensive Mediaeval wall paintings. A fragment of the 15th Century fresco is exposed to the left of the Screen.
The tower was built in the 14th Century and has substantial corner pinnacles connected to an octagonal spire by thin flying buttresses. The pinnacles and flying buttresses were rebuilt, and the spire repaired, in the late 1980s at a total cost of £90,000 with support from English Heritage and the Friends of St Peter’s. The clock was installed by the village in 1887 to mark Queen Victoria’s Golden Jubilee.
The chancel was about forty feet longer at the dissolution of the Priory. It was reported to the Archdeacon in 1578 that “…the cancell wyndowes are in decaye by the default of Mr Farrar”; presumably it was shortened soon after this date. One original lancet window survives on the south side.
Electricity was first put into the church in 1945. Much of the wiring survived until 1995, when the church was rewired and new lighting was installed. A new heating system was provided in 1994.
In the ancient churchyard at least 7000 inhabitants of Harrold are believed to be buried. The churchyard is now maintained by Harrold Parish Council, together with an adjacent modern cemetery.
The benefice of St Peter, Harrold was combined with that of St Mary, Carlton with Chellington in 1964.
There is a peal of six bells, rung every Sunday:
the Treble by Joseph Eayre of St Neots 1756;
the Second by Taylors of Loughborough 1898:
the Third and Fourth by Hugh Watts 1603;
the Fifth by John Hodson 1653;
and the Tenor by Chandler 1652 (recast by Taylors of Loughborough in 1898).
All the bells were rehung on a steel frame in 1898 in good time for a peal to be rung to celebrate the Relief of Mafeking in 1900. The Third bell was recast by Taylors in 1987 and the whole peal was rehung on modern bearings in 1989, financed by funds raised by local bellringers.