Smibert began drawing while apprenticed as a painter and plasterer, on moving to London he worked as a painter of coach carriages and a copyist. He studied under Sir James Thornhill at his academy, then travelled to Edinburgh and Europe seeking work as portraitist. He gained a reputation for his works copying old masters and receiving commissions for portraits in Italy and returned to England to capitalise on this.

Smibert painted a group portrait of the 'Virtuosi of London' society, of which he was a member; others in the group were John Wootton, Thomas Gibson, George Vertue, Bernard Lens, and other artists. He did not complete the painting, but did produce portraits in London up to September 1728, including one of Bishop Berkeley.

In 1728 he accompanied Berkeley to America, with the intention of becoming professor of fine arts in the college which Berkeley was planning to found in Bermuda. The college, however, was never established, and Smybert settled in Boston, where he married in 1730. He lived at the corner of Brattle Street and Queen-Street. He belonged to the Scots Charitable Society of Boston.

The Bermuda Group (Dean Berkeley and His Entourage), begun in 1728, finished 1739. Yale University Art Gallery

Plaque at Granary Burying Ground in Boston commemorating Smybert

In 1728 he began painting Dean George Berkeley and His Family, also called The Bermuda group, now in the Yale University Art Gallery, Yale University, a group of eight figures; it is maintained that the person farthest to the left is actually the artist himself. He painted portraits of Jonathan Edwards and Judge Edmund Quincy (in the Boston Art Museum), MrsSmybert, Peter Faneuil and Governor John Endecott (in the Massachusetts Historical Society), John Lovell (Memorial Hall, Harvard University), and probably one of Sir William Pepperrell; and examples of his works are owned by Harvard and Yale Universities, by Bowdoin College, by the Massachusetts Historical Society, and by the New England Historical and Genealogical Society.

In 1734, Smibert opened a shop where he sold paint, other artist's supplies, and prints. In his studio above the shop, he displayed casts and copies of Old Masters that he had painted in Europe. This collection, which Richard Saunders has termed America's first art gallery, provided much of the early artistic education for Charles Willson Peale, Gilbert Stuart, and John Trumbull.

Between 1740-42, he served as architect for the original Faneuil Hall, which he designed in the style of an English country market. The hall burned down in 1761 but was restored, and then in 1806 greatly expanded and modified by Charles Bulfinch.
His son Nathaniel was also a painter. Smybert lies in an unmarked grave in the Granary Burying Ground in Boston.