There is evidence to suggest that there were ancient settlements in the Claremorris area as there were a network of Crannogs established on the lakes in the locality, such as Mayfield and Rooskeybeg. However, Claremorris itself, takes it’s name from the Norman invader, Maurice de Prendergast who seized control of the area in south Mayo, where the town was developed. The castle located at Brize (Ancient Brize Castle), near Claremorris was the family residence of the Prendergasts from the 13th century onwards. The family were also responsible for the building of the Carmelite Abbey in nearby Ballinasmale during 1288. Although there is evidence to suggest that many people lived in the Claremorris hinterland from the 13th to the 16th century, it was during the late 17th century and early 18th century that the town area was founded. Many new buildings were constructed during the 18th century and in the early 19th century, there was a Roman Catholic Church built in the centre of Claremorris. This would be replaced a century later with the founding of Saint Colman's Church. The St. John’s Anglican Church was also built in 1828 and is currently utilised as the Town Library. In 1864, the great stately mansion, Castlemacgarrett House was built by the Browne family.
Towards the end of the 19th century, the town began to grow as the introduction of Railway travel helped to link Claremorris with Sligo, Galway and Ballinrobe. The Claremorris-Collooney Railway Line proved beneficial for freight transport at the time and in latter years was used for the carriage of sugar beet and it's by-products. Claremorris Railway Station had being opened earlier in 1862. With the formation of the Irish Free State in the 1920’s, Claremorris continued to grow due to it’s convenient location and because of the major rail infrastructure which the town boasted. In 1937, the Irish Meteorological Service established a Met Office on the outskirts of Claremorris, which complimented the Telephone Exchange which was already situated in the town. Claremorris has also produced some famous people, most notably, John Francis D’Alton, who rose through the ranks of the Roman Catholic Church to become the Primate of All-Ireland in 1946 and went on to take his place in the College of Cardinals in Rome during 1953. The great Irish Balladeer, Delia Murphy was also born in Claremorris in 1902, while Sir John Grey, the trusted and close friend of Daniel O’Connell was born on Mount Street in the south Mayo town.