William Winchester (1711-1790) purchased approximately 167 acres of land called White's Level in 1754 which became known as the city of Winchester. The Maryland General Assembly later changed the name of his town from Winchester to Westminster because Winchester was also the name of the county seat of Frederick County, Virginia where it was at that time located.

On June 29, 1864, the cavalry skirmish known as Corbit's Charge was fought in the streets of Westminster, when two companies of Delaware cavalry attacked a much larger Confederate force under General J.E.B. Stuart.

In April 1865, Joseph Shaw, newspaper editor, had his presses wrecked and his business destroyed, and was subsequently beaten and stabbed to death by four men in Westminster, allegedly because of an anti-Lincoln editorial that was published the week before the actual assassination. In a later trial at the Westminster Court House the four men were acquitted; the reason cited was "self-defense".

Since 1868, Westminster has held an annual Memorial Day parade, which is the longest continuously running Memorial Day parade in the country.

Just north of Westminster is the farm at which Whittaker Chambers hid the so-called "pumpkin papers."

A historic marker states that Westminster was the first place in the nation to offer Rural Free Delivery postal service.

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