He participated in Benjamin Franklin's mission to Paris in 1776-8, represented Maryland in the Continental Congress in 1778 and 1779 and was the principal diplomat for the United States to Spain from 1782 to 1794.

Carmichael was born sometime around 1739 at the family home (Round Top) in Queen Anne's County, Maryland, on the Chester River just opposite Chestertown.

But by the time the Revolutionary War began, he had decamped to London, England, and soon after, in 1776, made his way to Paris, carrying letters to the Continental Congress sewn inside the cover of a pocket dictionary.

When Jay returned in April 1782 Carmichael became Chargé d'Affaires, remaining in this post at the Spanish royal court until illness forced his replacement in 1794.

In 1792, President George Washington appointed Carmichael a commissioner plenipotentiary (together with William Short) to negotiate a treaty with Spain protecting American navigation rights to the Mississippi River.