Located to the southeast of the centre of Haddington, on the W bank of the River Tyne, St. Mary's is a building of cathedral-scale, indeed it is the longest parish church in Scotland. Completed in 1462, although begun in the late 14th Century and consecrated around 1400, St. Mary's is one of three great pre-Reformation churches in the Lothians, the others being St. Giles in Edinburgh and St. Michael's in Linlithgow. Known as the Lamp of Lothian such was its beauty and spiritual significance.
The church was badly damaged during the Siege of Haddington (1548-9). At the request of John Knox (1513-72), a son of Haddington, the nave was restored and used as the parish church for 400 years, while the choir and transepts remained open to the elements. A major restoration in the 1970s returned the church to its former glory. Today the church maintains a tradition of ecumenicalism, exemplified by the Lauderdale Aisle, an Episcopal chapel within the church, with strong Roman Catholic and Orthodox influences. This is the burial place of the Earls and Duke of Lauderdale. Other features of the church are its fine stained glass, including work by Alexander Ballantine (1841 - 1906) and William Wilson (1905-72), but most notable is the Great West Window by James Ballantine (1878 - 1940). The organ was installed within a purpose-built gallery in the North Transept in 1990, comprises twenty-four stops and was built locally by Lammermuir Pipe Organs at Oldhamstocks. New bells were installed in 1999, replacing those removed by the English army in 1548. Dedicated by the Rt. Rev. John Cairns, Moderator of the General Assembly of the Church of Scotland, the new bells came from Dunecht House, near Aberdeen. To keep them in Scotland, they were purchased for St. Mary's through grants, donations and National Lottery funding.