The book I read to research this post was The Secret Service by Philip H Melanson Ph.D. which is an excellent book which I borrowed from the library. The Secret Service has its roots in the 19th Century & its original role was to apprehend currency forgers. This type of forgery was a major problem & between a 1/3 & a 1/2 of all the US money in circulation was forged. At this time the USA was recovering from the Civil War & there were literally hundreds of banks in every state producing there own currency. The result of this was most people didn’t recognise all the different types of currency & even a fair forgery was likely to be accepted in most shops & banks. This resulted in currency production being streamlined into one type of currency which is still used. Sometimes when there were lots of banks producing organised criminals would bribe the staff to print money for them. The Secret Service used quite heavy handed methods to deal with these forgers.
A while after that there was a man called Alan Pinkerton who started the Pinkerton Detective Agency which was the 1st successful private detective agency in America. Anyway he uncovered a plot to assassinate Abraham Lincoln & warned him to change his itinerary which saved his life on that occasion. Lincoln interviewed him & was very impressed so he made him the head of The Secret Service & his mandate was to protect the president.
When Edgar Hoover was president a man who was just a harmless trespasser got past the security & got into the dining room of The White House. Hoover wanted to know how this could have happened & discovered The Security Service & the White House Police didn’t have a single coordinator to manage both of them. As a result of this the White House Police became the Uniformed Branch of The Secret Service. A system which exists to this day.