It is about two-thirds mile (1.1 kilometres) inland in a north-easterly direction from Dunnet Bay on the north coast of Scotland.

The south-east part of the loch is the deepest, reaching 7 ft (2.1 m).

[3] The overflow of the loch is through the Dunnet Burn, which exits from the south shore.

[3][4] There is a large population of wild fowl on the inland lochs around Dunnet, with Mallard, Wigeon, Pochard and Tufted duck being recorded at St. Mary's Loch.

The chapel's priests ascribed virtuous qualities to the water of the loch and took offerings at the altar from visitors using it, enabling curing of the sick and enriching of the church.