His handiness as a carver improved rapidly and he was employed for carved architectural details for many important late Federal and Greek Revival buildings in Boston, such as the Ionic and Corinthian capitals for the steeple of Park Street Church, built in 1810.

On November 2, 1825, Willard was chosen architect and superintendent of Bunker Hill Monument, his design having been accepted by the building committee in the following year.

Willard also invented the machinery to cut and handle the slabs of stone in what became known as the Bunker Hill Quarry, which evolved into a major industry for the town.

To get the cut slabs to a wharf on the Neponset River, a distance of two and three-quarters miles, the first commercial railway in the United States was built—the Granite Railway—over which, on the morning of October 7, 1826, the first horse-drawn cars passed, under the direction of a young engineer by the name of Gridley Bryant.

He was engaged in this work for seventeen years, being frequently interrupted by want of funds and by disagreements in the committee in charge, but on July 23, 1842, the top stone of the monument was laid, and on the anniversary of the battle in 1843 its completion was celebrated in the presence of the president of the United States, his cabinet, and a large concourse of citizens from every part of the United States.