Brumby is believed to be one of the oldest surnames in England, estimated at around 1130 years old. It is Old Norse in origin and, as most surnames, is derived from a place of settlement. In this case a small Danish farmstead, originally called Brúnibýr, in what was the ancient Angle Kingdom of Lindsey, now the north west of Lincolnshire.

In the autumn A.D. 866, what the Saxon Chronicle describes as "a great heathen army" attacked eastern England. Originating in what is now Denmark and Norway and under the command of Halfdan Ragnarsson and Ivar the Boneless it’s aim was to conquer and settle England.

The Great Army conquered Northumbria, quickly followed by the East Anglia in A.D. 870 and Mercia in A.D. 874. From there the Danes began to settle the lands they had conquered, where, according to the Saxon Chronicle, “they proceeded to plough and to support themselves.” The Danes had come to colonise much of the eastern coast of England in what had been the Angle kingdoms of East Anglia, Mercia, Northumbria and Lindsey. It is during these times that the settlement of Brumby is thought to have originally been made by agreement between Danish settlers and the Angles of Lindsey around the years A.D. 874 – 877. It was the beginning of what was to become the Danelagen, or Danelaw during which the language of what is now known as Lincolnshire was Old Norse and the customs and traditions of the Danes were observed.

Brumby is derived from the Old Norse personal name Brúni and the Old Norse word for a farmstead, býr. Brúni is found as both a personal name and as a by-name throughout Scandinavia. Brúni was one of the early descriptive names of the Norse God, Óðinn (Odin) and is also found as both a personal and by-name in the Icelandic Sagas.

Brumby is found in written form in the Domesday Book of A.D. 1086/87 as Brunebi, a clear derivative of Brúnibýr. The Domesday Book describes how the small settlement of Brunebi formed part of the manor of Kirton (in Lindsey) and consisted of 80 acres of meadow with 14 freeman and 3 ploughs. Prior to the Norman Conquest it was the property of Edwin, Earl of Mercia before passing into the hands of the Norman King, William.

The first recorded use of the name as a surname occurs in the Hundred Rolls of A.D. 1273 where a Peter de Bruneby and a Stephan de Bruneby are listed. Over the centuries, the name Brúnibýr evolved into Brunebi and finally into what is now Brumby.

The change from Brun to Brum is quite normal, occurring in many Brumptons and Bromptons, and derived names show the same change, for example, Richard Burnby of A.D. 1486 being followed by many relatives called Bromby and Brumby.