The United States has lost 60% of its career ambassadors since January, according to the head of the labour union that represents US diplomats.
"Leadership ranks are being depleted at a dizzying speed," Barbara Stephenson, head of the American Foreign Service Association (AFSA) chief, wrote.
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".
The decline of US diplomacy?
Analysis by Barbara Plett Usher, State Department correspondent, BBC News
This letter is the latest cry of distress from diplomatic veterans warning about the "mounting threats" to the State Department.
They say that the exodus of disillusioned top career officers is depriving the institution of decades of diplomatic experience.
And that the freeze on lower level hiring will weaken it in the future. Then there is the slow pace of political appointments to crucial jobs.
That has picked up lately but it's been hindered by the small pool of qualified personnel: President Trump refuses anyone who opposed his candidacy, and most of the Republican foreign policy establishment did.
Mr Tillerson is in the midst of reorganising and streamlining the department. Previously he's rejected claims that a staffing crisis is affecting the institution, listing the many international issues on which he's working and naming the career diplomats on whom he relies for help.
All of which leaves Ms Stephenson to wonder darkly about who's behind the alleged impetus to "weaken the American Foreign Service."
As well as envoys, the staff numbers of other high-ranking diplomatic ministers have fallen, reports the organisation, which was founded in 1924.
Mrs Stephenson writes that the decisions by career diplomats (who, unlike political appointees, do not change with every White House administration) was fuelled in part by the government's decision to "slash promotion numbers by more than half".
A hiring freeze has also led to a drop in entry level staffers joining the US agency tasked with worldwide diplomacy.
"The talent being shown the door now is not only our top talent, but also talent that cannot be replicated overnight," Mrs Stephenson writes.
She adds that if such an exodus were to occur within military ranks, "I would expect a public outcry".
Mrs Stephenson also points to Congress' opposition to such drastic cuts to the department, saying the Senate has labelled doing so "a doctrine of retreat".
In her scathing editorial, she questions "Why such a focus on decapitating leadership? How do these actions serve the stated agenda of making the State Department stronger?
"Where is the mandate to pull the Foreign Service team from the field and forfeit the game to our adversaries?"
Former Secretary of State Madeleine Albright - who was a staunch supporter of Hillary Clinton - told MSBNC on Wednesday that she is "deeply, deeply troubled" by the lack of staffing, "because you cannot be a major power in the world and not have a functioning diplomatic service".
A year after President Donald Trump's election, there are 152 senior posts unfilled at the Department of State,
according to the tracker by Washington Post and Partnership for Public Service.
Last week Mr Trump was asked a question about vacant positions at the State Department.
"I'm the only one that matters," he replied.
