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. The treaty was concluded shortly after Carmichael's death, and became known as the Treaty of San Lorenzo or Pinckney's Treaty, Thomas Pinckney having stepped in to complete the negotiations. Carmichael was also involved in negotiations to free American mariners who had been taken captive by the Dey of Algiers, a situation that later led to the Barbary Wars.