Baron Dorchester was a title that was created twice in British history, once in the Peerage of Great Britain and once in the Peerage of the United Kingdom. The first creation came in the Peerage of Great Britain on 21 August 1786 when the soldier and administrator Sir Guy Carleton was made Lord Dorchester, Baron of Dorchester (on Thames), in the County of Oxford. He was succeeded by his grandson, the second Baron. He was the son of the Hon. Christopher Carleton, eldest son of the first Baron. Lord Dorchester died unmarried at an early age and was succeeded by his first cousin, the third Baron. He was the son of the Hon. George Carleton, younger son of the first Baron. He had no sons and was succeeded by his first cousin, the fourth Baron. He was the son of Reverend the Hon. Richard Carleton, younger son of the first Baron. Lord Dorchester was a Colonel in the Coldstream Guards. He was childless and the title became extinct on his death on 13 November 1897.
The peerage was revived on 2 August 1899 when the Honourable Henrietta Carleton was made Baroness Dorchester, of Dorchester in the County of Oxford, in the Peerage of the United Kingdom. She was the elder daughter of the third Baron of the first creation. She was the wife of, firstly, Francis Paynton Pigott, and, secondly, of Major-General Richard Langford Leir. In 1899 she assumed by Royal licence the surname of Carleton. She was succeeded by her son from her first marriage, the second Baron. He was a soldier and fought in the Second Boer War and the First World War. He had two daughters but no sons and on his death in 1963 the title became extinct for the second time.
As indicated by the territorial designations, the titles referred to Dorchester in Oxfordshire (also known as Dorchester-on-Thames), and not to the more famous Dorchester, county town of Dorset.