According to Asser's biography of King Alfred, written in 893 AD, its old name Bearrocscir takes its name from a wood of box trees, which was called Bearroc (a Celtic word meaning "hilly"). This wood, perhaps no longer extant, was west of Frilsham, near Abingdon.
Berkshire has been the scene of some notable battles through its history. Alfred the Great's campaign against the Danes included the Battles of Englefield, Ashdown and Reading. Newbury was the site of two English Civil War battles: the First Battle of Newbury (at Wash Common) in 1643 and the Second Battle of Newbury (at Speen) in 1644. The nearby Donnington Castle was reduced to a ruin in the aftermath of the second battle. Another Battle of Reading took place on 9 December 1688. It was the only substantial military action in England during the Glorious Revolution and ended in a decisive victory for forces loyal to William of Orange.
Reading became the new county town in 1867, taking over from Abingdon, which remained in the county. Under the Local Government Act 1888, Berkshire County Council took over functions of the Berkshire Quarter Sessions, covering the administrative county of Berkshire, which excluded the county borough of Reading. Boundary alterations in the early part of the 20th century were minor, with Caversham from Oxfordshire becoming part of the Reading county borough, and cessions in the Oxford area.
On 1 April 1974, Berkshire's boundaries changed under the Local Government Act 1972. Berkshire took over administration of Slough and Eton and part of the former Eton Rural District from Buckinghamshire. The northern part of the county became part of Oxfordshire, with Faringdon, Wantage and Abingdon and their hinterland becoming the Vale of White Horse district, and Didcot and Wallingford added to South Oxfordshire district. 94 (Berkshire Yeomanry) Signal Squadron still keep the Uffington White Horse in their insignia, even though the White Horse is now in Oxfordshire. The original Local Government White Paper would have transferred Henley-on-Thames from Oxfordshire to Berkshire: this proposal did not make it into the Bill as introduced.
On 1 April 1998 Berkshire County Council was abolished under a recommendation of the Banham Commission, and the districts became unitary authorities. Unlike similar reforms elsewhere at the same time, the non-metropolitan county was not abolished. Signs saying "Welcome to the Royal County of Berkshire" have all but disappeared but may still be seen on the borders of West Berkshire District, on the east side of Virginia Water and on the M4 motorway. There are also county signs on the south side of Sonning Bridge on the B478, on the A404 southbound carriageway crossing the river Thames, and heading north on the A33 at the start of the dual carriageway just past Stratfield Saye.
A flag for the historic county of Berkshire was registered with the Flag Institute in 2017.