The United States Secret Service was the organisation responsible for protection of governmental officials, including the President.
The Players tried to start World War III in 1951 by mentally influencing Harry S. Truman in the White House, but were prevented when the Eighth Doctor informed the Secret Service of their plan. (PROSE: Endgame)
Around the late 1950s, the United States government hid a transmission from an alien species — in the Tenth Doctor's words, the Secret Service had "locked this transmission away in a box for their own ends". (PROSE: Blue Moon)
The Secret Service was startled when the Eleventh Doctor had River Song make the TARDIS, which was cloaked after landing in the Oval Office, visible. (TV: The Impossible Astronaut)