Willard was born in Petersham, Massachusetts. Willard trained as a carpenter with his father, a farmer who did carpentry in the winters; he went to Boston in 1804, working during the day and reading books of architecture and drawing in the evenings. His handiness as a carver improved so rapidly that 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 ; in the same year he carved the eagle for the pediment of the new Custom House. In 1818 he made a model of the capitol at Washington for Charles Bulfinch, who was then engaged on the Massachusetts State House, and later did several further works of this sort, among which were models of the Pantheon and the Parthenon for Edward Everett. From wood carving he turned to stone carving, and in 1820 was engaged on the Ionic capitals and other stonework of the Episcopal St Paul's Church, the first example of Greek Revival architecture in Boston By 1821 Willard had become so successful that he gave classes in architecture and drawing in his studio near St Paul's; there Horatio Greenough was a pupil. Willard added ship figureheads to his craft, from 1823.
He designed the Bunker Hill Monument in 1825, and construction began in 1827. Willard discovered satisfactory granite quarries for the stone at Quincy, and the granite for the monument came from there. 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.
Willard's work on the Bunker Hill Monument was succinctly recorded in 1849, by Amos Lawrence, secretary of the Building Committee, who wrote in the flyleaf of the committee's records: "Solomon Willard walked three hundred miles to examine granite quarries (Hallowell, Maine, and other places), gave a thousand dollars to the Monument Association, and worked like a dog for the association for years for merely his necessary expenses (which were very small), and is now at work at Quincy" (History 1926).
In Framingham, Massachusetts, Willard's First Baptist Church of 1826 still stands, now the oldest building in the town The Norfolk County Courthouse in Dedham, Massachusetts is also his work. In the same year he was also architect of Divinity Hall, Harvard Divinity School. Willard also designed the Greek Revival Framingham Village Hall. The Gothic Revival Mission Church of St. John the Evangelist on Bowdoin Street, Boston, dated to 1831, is also probably his design.
He is credited with designing some of the first hot-air central heating in an American building. In 1829 his recent pupil, the brilliant young architect Isaiah Rogers, designed the innovative Tremont House in Boston. This was the first American hotel to have indoor plumbing and it became the prototype of a modern, first-class American hotel.
Willard died in Quincy, Massachusetts. In 1865, William W. Wheildon wrote a Memoir of Solomon Willard, Architect and Superintendent of the Bunker Hill Monument published by the Massachusetts: Monument Association, which is the primary source for his biographers.