Lyman was born in Durham, Connecticut to Thomas Lyman. While attending Yale College, Lyman was commissioned as a captain in the Continental Army, serving in the battles of Ticonderoga, Crown Point, and St. Johns. After his graduation in 1776, he was commissioned as a major, served at the battle of White Plains, and from 1778 until the close of the war was an aide to General William Heath. He married Mary "Polly" Wanton in 1782 in Newport, Rhode Island, and they had 13 children. Lyman served as a member of the Hartford Convention in 1814-15 He later acted as surveyor for the port of Newport. He also practiced law, and served as the Chief Justice of the Rhode Island Supreme Court from 1812 to 1816. He retired north of Providence in 1808, and became a partner in the Lyman Cotton Manufacturing Company. He was also an original member of the Society of the Cincinnati. Lyman was elected a member of the American Antiquarian Society in 1815.