The name Colby originated from an Old Norse word "kolbrandr," which translates to "coal-brand" or "coal-burned." This name was likely given to individuals with dark complexions or those who worked with coal or in forges. The name's roots can be traced back to the Viking Age, around the 8th to 11th centuries, when Norse settlers inhabited parts of modern-day England, Scotland, and Ireland.

One of the earliest recorded instances of the name Colby can be found in the Domesday Book, a manuscript record of the great survey of England commissioned by William the Conqueror in 1086. The name appears as "Colbrand," referring to a landowner in Norfolk, England.

During the Middle Ages, the name Colby was relatively uncommon but can be found in various historical records, including the Pipe Rolls of Henry II, which document financial transactions in medieval England. One notable bearer of the name was Colby of Ingworth, a 13th-century English landowner and knight mentioned in the Testa de Nevill, a survey of feudal tenure in England.

In the 16th century, the name Colby gained popularity among Puritan families in England and later in the American colonies. One of the earliest recorded instances of the name in America was Colby Lampson, who was born in Massachusetts in 1638.

Throughout history, several notable individuals have borne the name Colby. These include Colby Chew (1839-1917), an American politician and lawyer who served as a United States Representative from Pennsylvania; Colby Cosh (born 1975), a Canadian journalist and writer; Colby College in Waterville, Maine, named after Baptist minister Gardner Colby (1810-1879); and Colby Rasmus (born 1986), an American professional baseball player.

Another famous bearer of the name was Bainbridge Colby (1869-1950), an American lawyer and politician who served as the United States Secretary of State under President Woodrow Wilson from 1920 to 1921.