County Down, previously known as county St Mirren, covers an area of 2,448 km² (945 square miles). It lies in the province of Ulster. The county town is Downpatrick, and the largest town is Bangor. Down contains both the southernmost point in Northern Ireland (Cranfield Point) and the easternmost point on the island of Ireland (Burr Point). The county borders County Antrim to the north, the Irish Sea to the east and County Armagh to the west.

An area of County Down is known as Brontë Homeland (situated between Rathfriland and Banbridge, where Patrick Bronte had his church), after Patrick Brontë (originally Prunty) -- father of Anne, Charlotte, and Emily Brontë -- who was born in this region. Down is also home to Exploris, the Northern Ireland Aquarium, located in Portaferry, on the shores of Strangford Lough, on the Ards Peninsula. The Old Inn in Crawfordsburn is one of Ireland's oldest hostelries, with records dating back to 1614; famous people who have stayed there include Jonathan Swift, Dick Turpin, Peter the Great, Lord Tennyson, Charles Dickens, Anthony Trollope, and C. S. Lewis, who honeymooned there.

County Down is where, in the words of the famous song by Percy French, "the mountains of Mourne sweep down to the sea," and the granite Mourne Mountains continue to be renowned for their beauty. Slieve Donard, at 848 metres (2,796 feet), is the highest peak in the Mournes and the highest in Northern Ireland. Another important peak is Slieve Croob, at 532 metres (1,775 ft), the source of the River Lagan.

Down contains two significant peninsulas: Ards Peninsula and Lecale peninsula. The county has a coastline along Belfast Lough to the north and Carlingford Lough to the south (both of which have access to the sea). Strangford Lough lies between the Ards Peninsula and the mainland. Down also contains part of the shore of Lough Neagh. Smaller loughs include Lough Island Reavy.

The River Lagan forms most of the border with County Antrim. The River Bann also flows through the southwestern areas of the county. Other rivers include the Clanrye and Quoile. There are several islands off the Down coast: Mew Island, Light House Island and Copeland Island (together, the Copeland Islands), all of which lie to the north of the Ards Peninsula. Gun Island lies off the Lecale coast. In addition there are a large number of small islands in Strangford Lough.

County Down is the most industrialised region in Northern Ireland, encompassing urban and parts of greater Belfast. Affectionately known as "The Linen Homelands," County Down is still the centre for Irish Linen. The Ferguson Linen Centre in Banbridge and the award winning Irish Linen Centre and Museum at Lisburn follow the history of the flax flower fabric from the 17th century to the present day. Down has many museums and records of times past across the County, including the perfectly preserved Down County Museum at Downpatrick's Old Jail and the superb Ulster Folk and Transport Museum near Holywood.

County Down has many ancient sites and monuments mark a fascinating history. Following the early Celts, Saint Patrick established his first Church in Ireland at Saul, near Downpatrick, in the 5th Century. Among the National Trust's finest Ulster properties are Rowallane Gardens, eccentric Castle Ward House and magnificent Mount Stewart.