The parish is bordered by Portland in the east, St. Ann in the west, and parts of St. Catherine and St. Andrew in the south. The parish covers an area of 610 sq km, making it Jamaica's fifth smallest parish. The terrain is mountainous, rising up to almost 4000 feet at the highest point, but there are no distinctive mountain ranges. There are three (3) main rivers in St. Mary; the Rio Nuevo River, Wag Water and White Rivers. These rivers are often slow, and at times create swamps in the surrounding area.

The capital town Port Maria, is a small rugged town was originally named "Puerto Santa Maria". It was the second town established by Spanish settlers in Jamaica. Several buildings have survived from Port Maria's glory days of being a busy port, including the St Mary Parish Church and the old courthouse. The town also boasts a beach (pagee) an island (Carbarita Island) and a beautiful winding coastline.

St Mary has had formidable challenges sustaining economic growth over the years. Indeed, the parish, for several years, has been listed as one of the poorest in Jamaica, although there is potential for vast development. The parish has a good variety of agricultural resources. The principal products are bananas, sugar, citrus, pimento, cocoa, coconuts, coffee, vegetables and breadfruit. It also boasts what is thought by some to be one of the best secondary level schools in the nation, St. Mary High, from which several outstanding people have emerged to occupy reputable positions in varying sectors at home and abroad.