The cathedral of St Martin was the parish church of St Martin until 1927. It is situated within the medieval centre, defined by the River Soar to the W and the town walls on the other sides, and consists of an aisled nave with an outer aisle on the S; N and S porches, a chancel with chapels and a crossing tower with a tall spire. The oldest, and the only Norman feature is a fragment of billet moulding in the S wall at the E end of the N aisle, i.e. in the N exterior wall of the original aisleless 12thc nave. VCH states that the only Norman work left in the church is a short piece of billet work on the N side of the NW crossing pier, and this is the same piece. The tower was Norman originally, but was rebuilt by Raphael Brandon in 1861-62. The N and S arcades are mid-13thc, although the N was rebuilt by Brandon in 1846-48, and there is a Perpendicular clerestorey, also rebuilt by Brandon. the outer S arcade is slightly later in the 13thc. The nave was extended westwards by one bay in the 15thc. The chancel chapels were rebuilt by Brandon in 1865. The cathedral is now best known as the burial site of Richard III, whose tomb was installed in 2015. It consists of a massive block of Swaledale fossil marble on a Kilkenny marble plinth and was carved by James Elliot, Gary Breeze, Stuart Buckle and Thomas Greenaway.

Leicester was a considerable city in 1086, with more than 300 houses listed of which Hugh de Grandmesnil owned 110, a further 24 in comn with the king and more than 60 pertaining to other manors he hld. The king held a further 39 house in his own right and Countess Judith was the next largest landlord with 28 houses. The Domesday Survey also records that High de Grandmesnil held 2 churches in the city without specifying which ones. Although St Martin's did not become a cathedral until 1927, it has always been the principal church in the borough. The advowson was probably one of those given to the College of St Mary de Castro in 1107 and transferred to Leicester Abbey in 1143. It remained with the Abbey until the Dissolution when it was transferred to the Crown.

The billet stringcourse is the only survival from the fabric of the Romanesque aisleless church. It appears to be in-situ, and can probably be dated between c. 1080 and c.1125.