As the U.S. Secret Service describes itself on its website:

We are one of America's oldest federal law enforcement agencies, originally created in 1865 to stamp out rampant counterfeiting in order to stabilize America's young financial system. By the end of the Civil War, nearly one-third of all currency in circulation was counterfeit. As a result, the country's financial stability was in jeopardy. To address this concern, the Secret Service was established in 1865 as a bureau in the Treasury Department to suppress widespread counterfeiting.

After the assassination of President McKinley in 1901, the Secret Service was tasked with the full-time protection of the President of the United States.  Over time, this protective mission has been expanded by statutory changes, Presidential Decision Directives, Homeland Security Presidential Directives, National Security Presidential Directives, and various Executive Orders.

The Secret Service currently protects 36 protectees—a reduction from the 42 it protected during Donald Trump's presidency. The agency employs about 8,000 people; this year it received $3.1 billion in congressional funding.

Since 1901, two secret service agents have been killed in the line of duty. William Craig was killed on September 3, 1902, when a trolley car rammed into the open horse-drawn carriage carrying President Roosevelt in Pittsfield, Massachusetts.

Leslie Coffelt was killed while defending President Harry Truman against an attempted assassination by Puerto Rican nationalists on November 1, 1950, at Blair House, where the president was living during renovations at the White House.