The Warren Commission formed after the assassination of President Kennedy produced this appendix on the history of presidential protection. They note that before Lincoln, "there was remarkably little concern about the safety of Presidents and few measures were taken to protect them." This persisted even after President Jackson survived an assassination attempt due to a misfiring gun. In the 1840s Congress did assign a guard to the White House — but apparently with more of an aim to protecting the building and grounds than its primary occupant.

During the Civil War Lincoln was guarded irregularly — sometimes by Washington, D.C. policemen, and sometimes by Army units. Lincoln "was reluctant to surround himself with guards and often rejected protection or sought to slip away from it." This, the report notes, "has been characteristic of almost all American Presidents."

The congressional report after Lincoln's assassination, "with traditional reluctance, called for no action to provide better protection for the President in the future." The Warren Commission notes that some people at the time saw Lincoln's assassination as a unique reflection of the Civil War, which was now over, and not something indicative of a need for broader protections. Indeed, for more than a decade after Booth, presidents continued to walk about in public alone and unguarded. (I remember, but cannot find a source, an anecdote about either Grant or Hayes, that upon receiving a threat on his life, the president didn't change his routine other than to purchase a heavier walking stick. Sadly, you should disregard this anecdote unless someone else can find a source for it, but it's delightful enough I thought I'd mention it as an aside.)

After President Garfield was shot, the New York Tribune contrasted "the simple Republican manner of life which the custom of nearly a century has prescribed for the Chief Magistrate of the United States" with "the forms and safeguards of courts" — implying that bodyguards were seen as a courtly, monarchical affectation, where as a republican president should be open and accessible to the people (July 3, 1881, quoted in Warren Commission report). Surprisingly or not, despite the second assassination of a president in two decades, Congress "took no steps to provide the President with personal protection."

The Secret Service, initially a group for combatting counterfeiters and financial crimes, first began providing part-time presidential protection in 1894 — apparently a coincidence because a group of Colorado gamblers were suspected of plotting to assassinate President Cleveland. President McKinley actually had Secret Service protection when he was assassinated, but it was his death that finally spurred Congress to give the Secret Service "full-time responsibility for the safety of the President."

Returning to Lincoln, he was certainly aware that many people wanted to kill him. President Theodore Roosevelt later quoted Lincoln saying, "Though it would be safer for a president to live in a cage, it would interfere with his business." He had sporadic guards but no formal protection.

Fascinatingly, General Ulysses Grant was originally invited to attend Ford's Theater with Lincoln on that fatal night. He begged off with an excuse about needing to be with his children, but possibly because Julia Grant and Mary Todd Lincoln hated each other.

From a recent biographical article on Grant:

Grant would never forgive himself for begging off, certain that had he accepted the invitation, his bodyguards stationed outside the door would have stopped Booth. As lieutenant general of the U.S. Army he was entitled to armed protection around the clock. The president in those days was not.

Hopefully this was helpful!