Originally known as Druid Mill, Union Mill was built between 1865 and 1872. At the time, it was the largest cotton duck mill in the United States. A unique feature of the mill's construction is the use of locally quarried stone. The other mills in the area were constructed with brick.

Druid Mill was was the first mill in the area to feature a clock tower, which was clearly visible to the workers living in Druidville located across Union Avenue. The mill joined the Mount Vernon Woodberry Cotton Duck Company in 1899, which had a virtual monopoly on the world's production of cotton duck. The mill was then renamed Mount Vernon Mill No. 4.

Today, the mill is home to residences and businesses, including Artifact Coffee.

1500 Union Avenue, Baltimore, MD 21211