Mills followed his mentor Hoban to Washington, D.C. after he received a commission for design and construction of the White House in the new capital.

Also in 1811, Mills was involved in a significant renovation and remodeling of [Old] St. Mary's Church in Burlington, New Jersey, including the addition of a new semi-octagonal apse on the east end of the building.

Moving to Baltimore, Maryland, Mills designed St. John's Episcopal Church, the Maryland House of Industry, the First Baptist Church of Baltimore at South Sharp and West Lombard streets in 1817, and a Greek Revival mansion at the northeast corner of West Franklin and Cathedral streets (across from the Old Baltimore Cathedral/Basilica of the Assumption of Mary).

The mansion was later occupied from 1857 to 1892 by the Maryland Club, a dining and leisure society of Southern-leaning gentlemen.

Mills designed the nation's first Washington Monument, located in Baltimore with four surrounding park squares.

Howard was a Revolutionary War commander of the famed "Maryland Line" regiment of the Continental Army.

In 1820, Mills was appointed as acting commissioner of the Board of Public Works in South Carolina.

He also designed portions of the Landsford Canal in Chester County, on the Catawba River in South Carolina.

When a fire broke out in the Kingstree, South Carolina Building, which he designed, the county records on the first floor were protected due to his fireproofing measures.

This was the dominant style of building that was winning architectural design competitions and major projects of the time, both in Europe and in the United States.

Apart from stylistic movements in architecture going on in the world, Mills was involved in the more local context of building in the Mid-Atlantic States.

To build as Mills did on what is now the National Mall, he had to contend with the strictures of the plan by Pierre Charles L'Enfant, as well as Andrew and Joseph Ellicott.