Adjoining the north-west shore of Lough Neagh, the county covers an area of 2,118 km2 (818 sq mi) and today has a population of about 252,231.

The most significant site however is Mountsandel, located near Coleraine in County Londonderry is "perhaps the oldest recorded settlement within Ireland".

Towards the close of the reign of Elizabeth I their territory was seized by England, with the purpose of checking the power of the O'Neills, and was made the county of Coleraine, named after the regional capital.

Whilst The Irish Society as a whole was given possession of the city of Londonderry and Coleraine, the individual companies were each granted an estimated 3,210 acres (5.02 sq mi; 13.0 km2) throughout the county.

To the east and west, the land falls into the valleys of the Bann and Foyle rivers respectively; in the south-east, the county touches the shore of Lough Neagh, which is the largest lake in Ireland; the north of the county is distinguished by the steep cliffs, dune systems, and remarkable beaches of the Atlantic coast.

The county is home to a number of important buildings and landscapes, including the well-preserved 17th-century city walls of Derry; the National Trust–owned Plantation estate at Springhill; Mussenden Temple on the Atlantic coast; the dikes, artificial coastlines and the bird sanctuaries on the eastern shore of Lough Foyle; and the visitor centre at Bellaghy Bawn, close to the childhood home of Nobel laureate Seamus Heaney.

In the centre of the county are the old-growth deciduous forests at Banagher and Ness Wood, where the Burntollet River flows over the highest waterfalls in Northern Ireland.

The Londonderry and Lough Swilly Railway continued as a private bus company based in the city but operating predominantly in County Donegal until it closed in 2014.

The EA is divided into sub-regions: For Catholic grant-maintained schools administration is by the Derry Diocesan Education Office.

Two major centres of the University of Ulster are in the county, including its headquarters at Coleraine and the Magee Campus in Derry.

play in the Premier Division of the League of Ireland after leaving the Northern Ireland structures in 1985, having resigned from the Irish Football League at the height of the Troubles because of not being allowed play their home games at the Brandywell due to security concerns from other clubs.

The Northern Ireland Milk Cup was established in 1983 and is regarded as one of the most prestigious youth football tournaments in Europe and the world.

Cricket is particularly popular in the north-west of Ireland, with 11 of the 20 senior clubs in the North West Cricket Union located in County Londonderry: Limavady, Eglinton, Glendermott, Brigade, Killymallaght, Ardmore, Coleraine, Bonds Glen, Drummond, Creevedonnell and The Nedd.