Rudy Giuliani is a first.
As a little history, US security clearances began in 1939 via the Hatch Act, which required that government employees not be a member of organizations that desired to overthrow the US government.
There have been no non-cleared, non-elected people since 1939 that have served in such an important role as Giuliani. Even those that served before 1939 were thoroughly vetted by the military or Congress, and the vast majority had been specifically confirmed as a cabinet post by Congress.
As central policy-making by the White House carries security issues with it, all those who negotiate on behalf of the US government must be cleared to deal with the secrets, tactics, and desired results that such a role would naturally require.
As one cannot adequately prove a negative, I will disprove several other offered examples.
- Wild Bill Donovan had major military posts, was the assistant
Attorney General of the United States to Harlan Stone, then John
Sargent, serving in the antitrust division of the DOJ. This is a
man with the highest of security clearances when he was an adviser
- Vernon Walters served as official interpreter for Truman,
receiving a very high security clearance to do so, as he was having
to translate detailed treaty language. When he smuggled Kissinger in
and out of North Vietnam, he was doing so as a military attache,
utilizing his high military rank as security clearance for his
diplomatic work. Later, Congress confirmed Nixon's appointment of
him as Deputy Director of the CIA, which obviously is well beyond
any need to justify clearance-wise.
- James Monroe was a founding father, involved in high levels of
both state government, a position as a Senator, and a
Senate-confirmed minister (diplomat) to France before he assisted
Livingston in the Louisiana Purchase. While there were no security
clearances back then, he was approved by Congress as a lawful
official of the US government.
- Sidney Blumenthal was a senior adviser to Bill Clinton, and
thus had to receive Top Secret security clearance to do so. He left
government in 2001 in disgrace, and has not served in any sort of
ambassadorship or diplomatic role since that point. As a member of
the Clinton Foundation in 2011, he sent one-way memos (that turned
out to be extremely bad intel) about Libyan president Gaddafi
to Hillary Clinton, but there was no evidence that Clinton provided
him with intelligence or gave him responsibility for negotiation of
These were all men who had either undergone strict security checks via the US military or the US government for their roles long before they were asked to serve their country in less-formal settings. Rudy Giuliani has absolutely no security clearances, nor has he ever at any point in his life, even immediately following 9/11.