Ms Stephenson, a former ambassador to Panama, says there has been a "decapitation" of top talent.
The BBC has asked the Department of State for a comment.
"The rapid loss of so many senior officers has a serious, immediate, and tangible effect on the capacity of the United States to shape world events," Ms Stephenson writes.
The former ambassador to Panama writes in a forthcoming issue of the Foreign Service Journal that the art of diplomacy must be defended under an administration that she argues appears bent on "dismantling government as we know it".