It is a cruciform building, exhibiting features of neoclassical and baroque architecture, with an elaborate east-facing facade, and north and south transepts with stained glass windows.

Originally designed by Walter Lovi and William Robertson in the early 1830s, its large copper dome was added in 1916 by Charles Ménart, who also remodelled the interior.

[3] Incorporating elements of neoclassical and baroque design,[1][3] it has a cruciform shape, with an elaborate east front, made of polished ashlar sandstone,[3] and rubble in flanks and rear.

The side bays, topped with volutes and finials, have corniced central panels which have inset round-topped niches housing statues.

[1] Behind the east front, where the transepts meet the nave, is a large, octagonally faceted copper dome,[2][3] with oval windows in each face.

[2] The classical Greek cross-form interior is dominated by the central dome, which is painted with a pattern of gold stars on a dark blue background.

[2][3] At the east end of the nave, close to the entrance, is a small gallery, housing a pipe organ, which is accessed via a marble-floored vestibule.

[5] The construction was paid for using money collected by Lovi on a European fund-raising trip, during which Charles X, the King of France, donated the painting by Dubois of St Thomas, which forms the altarpiece.