The consecration cross of this, the north chapel, can be seen on the east wall. A certificate in the Public Records Office shows that 'Andrew de Dale and Robert Webster, Guardians of the Guild of the Holy Trinity in the Church of St. Andrew in the town of Harlaxton, certify to provide three tapers of wax to burn on certain days before the image of the Holy Trinity’.
In the north-east angle of the chapel is a remarkable stone niche cut out of a single block of stone. It rises from a corbel, in a square form like a lantern. The upper portion is filled with tracery, above which is a cornice and embattled parapet. It is suggested that this niche was for the light maintained by the Guild of the Holy Trinity. The image may have been made of stone and not considered to be papistical at the time of the desecration of the church.
By the piscina in the south wall of the chapel are some interesting carvings or scratchings. On the lower floor at the entrance to the chancel are the imprints of brasses to William Strood and Agnes his wife, who are buried in the Trinity Chapel and who later founded the Chantry Chapel.
Under an ornamented gothic arch on the north wall behind the organ, are the full length figures in alabaster with animals at their feet. No date or inscription is given and the male figure could be John Bluett who was a Lord of the Manor of the 15th century, a Ricard from Herefordshire, or a Sir William Rickhill from Kent. His black marble tomb dated 1656 is that of Sir Daniel de Ligne, who bought the Manor after escaping from religious persecution in Flanders. He was knighted in 1619 and was High Sheriff of Lincolnshire in 1631.
Also on this raised part of the north chapel is an octagonal print of stone which shows where the font originally stood. The only pieces of pre-reformation glass left in the church can be seen in the top of the windows behind the organ above the alabaster figures.