Castlebar, Mayo's county town, is located at the junction of roads from Westport, Claremorris, Foxford, Newport, Ballina and Erris.  Castlebar gets its name from the castle built by the Norman De Barry family in the 13th century. The town developed around the castle, which was devastated by the fire in 1798.

Among Castlebar's most engaging features is The Mall - a congenial, open green, shaded with trees that are over a century old, and used as a public space for arts, leisure and sports events. In 1888, George, the fourth Earl of Lucan, donated the green, once a cricket pitch belonging to the Bingham family, to the people of Castlebar. Some of the most important public buildings are situated around The Mall, such as, The former Methodist Church, the foundation stone of which was laid by John Wesley in 1785. On the east side is Daly's Hotel, where Michael Davitt founded the Land League in 1879. The Courthouse, on the north side of The Mall, built in 1820 and possibly designed by George Papworth - a British architect who practised mainly in Ireland during the nineteenth century. On the west side of  The  Mall, a 1798 commemorative monument and a memorial tombstone for John Moore, President of the 1798 Connaught Republic are both situated.  Overlooking The Mall from the west side are the modern headquarters of Mayo County Council, opened in 1989 on the former site of the Mayo County Infirmary.
Castlebar was originally a busy market town, the central point of which was Market Square. Saturday was market day and agricultural produce from the surrounding area was sold there right up to the 1960s. The market continues today on a minimized scale. Many large supermarkets have now located in Castlebar, making it a busy shopping town.
In addition to being a commercial centre, Castlebar was once a powerful garrison town,  with an imperial military barracks and a separate cavalry barracks on the site of the present Garda Station. The military brought meaningful additional income to the town, as well as diversity. They introduced new leisure activities, popularising sports such as rugby and cricket, and altered fashions in clothing, household goods and furnishings. The records from Wynne's Circulating Library and Newsagency suggest that they also supported newspapers and other reading materials, which would not otherwise have circulated in the town. The military barracks in Castlebar, Westport and Ballinrobe were formed on similar plans and layout, and all built in classical style. The Castlebar Army Barracks was built in 1834 to house 1,000 men, in 1891 the soldiers billeted and the population of Castlebar expanded to around 3,558 people. The Barracks burned during the Civil War, but was then rebuild after World War II and is currently used by Fórsa Cosanta Aitiúl, the reserve defence force.
Castlebar's town gaol experienced a lot of executions such as George Robert Fitzgerald's in 1786, Fr. Conroy of Lahardane in 1798 and Edward Walsh's on 19 August 1873. The goal was eventually destroyed in 1932 and the original buildings of the General Hospital were then built on the site.