The church of the Holy Trinity is located next to the castle of Millom, situated around 6 miles north of Barrow-in-Furness, Cumbria. It consists of chancel, nave and S aisle. There is a modern S doorway off the chancel, with medieval window heads re-set above it, remains of a (now-blocked) S doorway off the nave, and a N main doorway, this with a porch over it. There is also a W bellcote and a blocked arch at the W end of the nave, the purpose of which is uncertain. During a siege in 1644, parts of the church were extensively damaged. Although some remedial work was carried out thereafter to make the church serviceable, it was not until 1858 that a full restoration was undertaken. In 1930, the chancel was widened, at which time two early, carved stone fragments were found within the N chancel wall. These were then reused as part of the NE exterior corner of the chancel.

Millom does not appear in Domesday Book.

The Lordship of Millom was held by the Barony of Egremont/Copeland, but was in practice an independent lordship. William le Meschin, who had been given the barony by Ranulph le Meschin about 1120, then divided his barony into several manors, one of which was Millom. The first lord of Millon appears to have been Godard Boyvil/Boyville, who received this manor about 1130. The family were thereafter referred to as ‘de Millum’. The Boyvilles continued to hold the lordship until the reign of Henry III (1216-72), when Joan de Millom married John Hudleston. The Hudlestons retained the title until the 18th century, at which time it was bought by James Lowther. In the 14th or 15th century, the S aisle of Holy Trinity Church became a memorial chapel of the Hudlestons, some of whom were buried there.

The church of the Holy Trinity at Millom was rectorial until 1228, at which time it was given to Furness Abbey. One moiety was reserved for the Archbishop of York, and in 1230 it was assigned to the maintenance of three chaplains, with clerks, to the chantry ordained at St Nicholas’ altar in the cathedral of York. In the 13th century a S aisle was added to the nave and a new E window installed. During the 14th century, the S aisle was enlarged, which included new windows. During the medieval period, Holy Trinity church served as the mother church to several local parishes.

The only stylistically-datable Romanesque feature of the church is the N doorway, which is late 12th century, likely to have been carved sometime between 1160 and 1200.