The earliest sections of the church are 12thc. This church appears originally to have consisted of a rectangular nave and rectangular chancel. Carved work from the 12thc. survives on the baptismal font, S doorway, a grave cover and a small chancel window. Various additions were made in the 13thc. and the W tower constructed in the 14thc. Fragments of an early cross were found in excavations carried out in 1880. The N aisle was built in 1882.

Evidence for an early religious site is given by carved stonework which the Corpus of Anglo-Saxon Sculpture assigns to the period 10th-to-11thc. In 1092, King William II went into Cumbria, which he claimed, and set up a fortress in Carlisle. The upper section of Cumberland was called Allerdale below Derwent (i.e. Allerdale above the River Derwent), which came into the possession of Waldeve son of Goospatric. His son Alan, who succeeded Waldeve, gave half of Dearham to Simon Sheftling (or Scheftelynges), whose descendants took the name de Dereham, and the other half to Dolfin, son of Gospatric. William of Dearham appears as witness to an early-13thc. document of Alice de Romilly, daughter of William fitz Duncan(Cart. Gyseburne, p. 320, no. MCXLIII). William fitz Duncan had became heir of Allerdale through his mother Octreda, one of Earl Alan’s sisters, as Alan had died without a direct heir. In another early-13thc. charter, among the witnesses were William of Dearham and Nicholao de Scefhtl[ing] (Reg. of St Bees, p. 395 no. 390).There are no known documents referring to the church at Dearham until Alice de Romilly, in another document of the early 13thc., gave the church of Dearham to Guisborough Priory. (Monast. Anglic., p. 271 no. XV). The church remained attached to Guisborough until the Reformation. During the reign of King Henry III (1216-72) there was an attempt made by Thomas de Multon to transfer the advowson to Calder Abbey, but this failed. The church of Dearham was rated in the Taxatio Ecclesiastica of about 1291 at £13 6s 8d. After the dissolution of the monasteries, the advowson of Dearham’s church was given to the Bishop of Carlisle by Queen Mary.

Hyde and Pevsner suggest a date of about 1170-90 for the S doorway. They also compare the baptismal font with carved work at Fishlake, Yorkshire, which they date to about 1170. In their opinion, the Adam stone ‘may be as late as the font’. Stephens suggested 850-950 for the Adam Stone, but Collingwood did not believe it could be so early and placed it in the 12thc. Chapman has suggested that it is early 12thc. The baptismal font, however, is generally agreed to be of 12thc. date. Comparisons with animal heads, use of arches and interlace can be made between the font and the Adam stone.

Earlier writers have referred to two Norman lights in the S wall of the chancel. Although the westernmost one has a circular top, it is considerably larger than the middle window and does not now appear to be of Norman date. The easternmost window in the S wall is later and has modern stonework. It is in the eastern part of the chancel which seems to be a later extension. Cox and others state that the main part of the chancel was built in the 13th century but give no reason for this.

The capitals of the S doorway at Dearham are quite unusual. Stylistically, they are related to a capital of the S nave arcade of St Bridget’s Church in Brigham, which is located a short distance south of Dearham. Dearham's doorway should also be compared with the W doorway of the church of Abbey Cultram.