The part of Belfast south of Queen's University between the Malone and Lisburn roads, known as the 'Malone Ridge', underwent rapid expansion from the middle of the 19th century, as a prosperous and fashionable suburb of large detached villas and grand terraces.

Other notable examples of their then recent work in the city included Clarence Place in May Street (now occupied by Lambert Smith Hampton),[3] Moore and Weinberg's warehouse in present-day Donegall Square North (now housing the Linen Hall Library)[4] and the main building at Queen's University (now called the Lanyon Building).

St Thomas's is one of the grandest and most fully finished examples of High Victorian Gothic ecclesiastical architecture, not only in Belfast, but in Ulster.

In 1888 the church was enlarged at the south end, to a John Lanyon design, when the southwest porch was added, as well as the internal gallery with its Gothic timber stairway.

The interior with its tall, open timber-trussed roof is decorated with string courses and brickwork of contrasting colour, as well as carvings and mosaics.

Elegant features, such as the narrow Gothic windows in the chancel and the slender timber trusses, mingle with the robustly carved foliage which adorns the capitals to the nave columns and the black-banded red brick arcade itself.

The Hill organ of 1874 was enlarged by its builder in 1906, but has remained essentially unaltered since then, which makes it an instrument of considerable and national significance.