The "Iron Lady" had the pseudonym "Mrs Stone" when travelling to the United States, newly-released official documents show.
A declassified US Department of State file shows Margaret Thatcher travelled under the name in 2002 when flying into Miami.
Mrs Thatcher remained popular in the US long after her resignation as prime minister in 1990. Following her death in April 2013, US president Barack Obama paid tribute, saying many Americans will never forget Mrs Thatcher "standing shoulder to shoulder" with Ronald Reagan during his presidency in the 1980s.
Mrs Thatcher's pseudonym was disclosed after a Freedom of Information request by Shawn Musgrave, projects editor at website muckrock.com.
The document released on the website shows a message from the US embassy in London to the US Secretary of State, listed under the subject "protective security - travel of Margaret Thatcher to the US".
It states: "Margaret Thatcher will travel to the US on 12/4/02 arriving at Miami International Airport on Flight BA 2013 1815 under the name of Mrs Stone. She will be accompanied by two armed police officers."
The names of the detective sergeant and detective constable are redacted.
In his FOI submission, Mr Musgrave requested from the State Department: "All records on, about, mentioning or concerning Margaret Hilda Thatcher, Baroness Thatcher, nee Roberts (13 October 1925 - 8 April 2013)."
The search was limited to documents created from January 1 2001.