The Chapel was built by 1366 at the south-west corner of the main court, next to the hall on the west side. On 30 May 1352 the Bishop of Ely had given licence to the Master and Fellows to build a suitable Chapel or oratory to be served by a chaplain without prejudice to the parish church of St John Zachary.

The founder gave certain books and ornaments for a chapel and directed his executors to give a large number of more. In August 1366 Pope Urban V granted the petition of the Master and Fellows to celebrate mass and other divine offices 'in the chapel built and founded within the same'. There are no records of the building operations, but an account survives of small repairs in 1513, and a piscina discovered in 1864 behind plaster is probably a part of the original fittings. The earliest monument is the fine full-length brass, originally over the grave of Walter Hewke in the centre of the Chapel but removed in 1730 to the ante-Chapel.

The original Chapel must have been richly decorated with the ornaments provided by the founder for the three altars. Nothing is known of the effect of the Reformation upon its appearance but in 1643 the Puritan William Dowsing, charged by Parliament to remove all traces of popery in the eastern counties, found nothing to destroy save one inscription 'orate pro anima' on a gravestone. In 1729-30 the Chapel was completely redecorated under the auspices of Sir Nathanael Lloyd, Master, who converted it into what Cole called 'a neat and elegant small room, more like a Chapel of a Nobleman's family than of a Society'.

The new wainscotting extended at the east end up to the ceiling and enclosed a large oil painted by Stella, bought in Flanders and given by one of the Fellows; the ceiling was arched in brick with stucco and adorned with 15 coats of arms on bosses; the painted glass was removed, the floor was paved with white marble, and several gravestones and brasses were removed to the ante-Chapel. Lloyd had a vault dug under the Chapel for his own grave and gave directions for his own wall monument, which was erected after his death in 1741 opposite the simpler marble tablet which had been set up about 1708 in memory and at the wish of Dr Eden.

In 1864 the length of the Chapel was increased by eight feet eastwards, and in 1922 a room over the ante-Chapel was converted into a gallery in order to house an organ and introduce music for the first time. In 1957 Stella's picture was replaced with Manzuoli's Salutation (c 1570) on permanent loan from the Fitzwilliam Museum, minor alterations being made in the panelling to accommodate it. The painting depicts the pregnant Mary's visit to Elizabeth, mother of John the Baptist, as recounted in the first chapter of Luke's Gospel. In other respects the chapel has still kept the general appearance given to it in 1730.

Underneath the 18th century façade, the walls of the original medieval building remain in place. There are a few signs left of the Chapel's antiquity. The piscina can be found behind a hinged door in the officiant's stall, which is close to the east end of the Chapel, on the right facing the altar. There is a pointed arch in one corner of the ante-Chapel, and there are medieval buttresses on the outside south wall of the Chapel (in South Court). Otherwise, Nathanael Lloyd's refurbishment was comprehensive, and survives largely intact, with the exception of the 1864 extension eastwards of the Chapel, which created a raised platform for the altar and would have reflected the mid-19th century Anglican revival of interest in eucharistic worship, and the organ gallery created in 1922. There is a separate brief guide available to the 15 coats of arms on the Chapel ceiling.

Little indeed has been added to the Chapel since Nathanael Lloyd's alterations, but the modern visitor might like to notice in particular the two stained-glass windows designed by John Heywood and installed in the ante-Chapel in 1980 to mark the elevation of Robert Runcie, a former Dean of the College, to the See of Canterbury. Today the Chapel continues to serve the College community as a whole, with regular services during term time and weddings, baptisms, memorial and other services as occasion demands throughout the year. Worship is in accordance with the rites of the Church of England, but members of all Christian churches and of none are warmly welcome to attend. The Chapel continues to support a choir and organ scholars.

http://www.trinhall.cam.ac.uk/about/chapel/history.asp