The United States Park Police has a long history. Just how long is a matter of interpretation. Although the present organization dates from after the Civil War, its functional lineage may be traced nearly to the beginnings of the national capital in the 1790s.
In 1791 Pierre Charles L'Enfant completed his plan for Washington, and three commissioners appointed by President George Washington to bring the federal city into existence began to purchase the land needed for its public buildings and grounds. As construction proceeded on the Capitol and White House during that decade, the commissioners may have hired watchmen to guard those building sites. Surely they employed watchmen for security at the completed buildings and their grounds after the government moved from Philadelphia to Washington in 1800. Inasmuch as the commissioners' office was the direct ancestor of the National Park Service with respect to its national capital parks functions, these watchmen may be considered the functional ancestors of today's park police.
The new capital city was initially policed by a constable appointed by Prince George's County, Maryland, having been laid out on land previously within that jurisdiction. An 1802 act of Congress to incorporate a city government gave the city corporation authority "to establish night-watches, or patroles," and the next year the city council created the position of police superintendent, replacing the Prince George's County constable.  This may be considered the beginning of today's Metropolitan Police Department (which achieved that title in 1861 when Congress unified the separate forces for Washington City, Georgetown, and Washington County in a single Metropolitan Police District). 
From 1802 to 1867 Washington's federal buildings and grounds were under a Superintendent and then a Commissioner of Public Buildings, who reported to the Secretary of the Interior after the Interior Department was established in 1849. Initially the city's police regulations did not cover federal property, so offenders there could be prosecuted only "by the slow process of a writ of trespass." That difficulty was overcome in 1834 when Congress provided "that the regulations of the city of Washington for the preservation of the public peace and order, be extended to all the public buildings and public grounds belonging to the United States within the city of Washington whenever the application of the same shall be requested by the commissioner of the public buildings." The request was promptly made, and persons injuring or destroying federal property, driving carts and wagons on footways, and other such offenses could now be readily punished.  Presumably the city police were expected to apprehend most violators, for although records of the period specify appropriations for watchmen at certain public buildings, including the White House and Capitol, they contain no reference to watchmen in the parks or grounds.
An 1850 appropriations act funded "two watchmen to be employed in preserving the public grounds about the Capitol," and successive appropriations through the 1850s provided for watchmen at the Capitol and White House grounds and laborers on the public grounds and in the President's garden.  The Commissioner of Public Buildings had two day watchmen in the Capitol Square, two night watchmen at the White House, and a night watchman at the public stables on his rolls in the mid-1850s. 
The first official reference to watchmen elsewhere on the public grounds appears in 1866. In that first year after the Civil War B. B. French, the Commissioner of Public Buildings, responded to a congressional resolution that he identify his employees and attest that none were Confederates or Confederate sympathizers. His list contained 30 Capitol police, 5 Capitol watchmen, 2 watchmen at the White House, and 2 watchmen in Reservation 2, the Smithsonian Grounds. The last two, in the area now better known as the Mall and Washington Monument grounds, were also listed as "laborers."  Two successive appropriations acts that year provided for three more watchmen at the Smithsonian Grounds and one at Franklin Square. 
Soon afterward the watchmen came under military control. An act of Congress approved March 2, 1867, abolished the position of Commissioner of Public Buildings and assigned its duties to the Chief Engineer of the Army. The Chief Engineer, a general officer, would discharge this responsibility through an office titled Public Buildings and Grounds, headed by a field grade officer (normally a colonel). As if to reinforce the military connection, Congress decreed that "hereafter no person shall be appointed as policeman or watchman who has not served in the army or navy of the United States, and received an honorable discharge." The appropriations act containing this requirement and successive appropriations through 1870 continued to provide for five watchmen on the Smithsonian and Washington Monument grounds and one at Franklin Square. 
A half-century later, in 1916, the officer in charge of Public Buildings and Grounds traced his park police organization to the two park watchmen on the Washington Monument grounds when the Chief Engineer took over in 1867.  A published account of the force in 1931 repeated this understanding: "[I]n 1867 the forerunners of the present body of United States Park Police, consisting of two men appointed as watchmen for the Monument grounds, were appointed at a salary of $720 per annum. From that small beginning, the force has grown steadily with the park system of the Capital . . . ." 
As more of L'Enfant's squares and circles underwent park development in the postwar decades, more watchmen were needed to oversee them. The Chief of Engineers outlined their purpose in his 1879 annual report to the Secretary of War:
Appropriations for fiscal year 1875 covered four watchmen on the Smithsonian Grounds, one at Franklin Square, one at Lafayette Square, one at Lincoln Square, one at Washington Circle, and one at the circle at Massachusetts and Vermont avenues (later Thomas Circle). Those at the Smithsonian Grounds and Franklin and Lafayette squares received annual salaries of $720, the others $540.  By 1880 the force had grown to 12, listed as follows by badge number:
The badges, together with batons and police whistles, were issued after Congress, in an appropriations act approved June 15, 1880, directed that "each of the watchmen herein provided for shall have the same duties and powers of the Metropolitan police." [14 ] George H. Brown, who then over saw the watchmen in his capacity as public gardener, dispensed these items and kept a whistle for himself. Maj. William G. Brock, superintendent of the Metropolitan Police, responded with a general order in support of the watchmen: "Their calls for assistance will be responded to as quickly as if the calls were given by a member of the regular force." Thomas P. Meagan, representing the D.C. Commissioners, promised Col. Thomas L. Casey, head of Public Buildings and Grounds and Brown's boss, that Major Brock would "confer with you fully in reference to your 'Park Police' and our connection with them." 
From that time on the watchmen were regularly known as "Park Police," even though the designation did not become official for nearly 40 years. Their new authority originally extended only during the fiscal year covered by the appropriations act granting it, but Congress renewed this authority for the next fiscal year and then made it permanent in an appropriations act for fiscal 1883, approved August 5, 1882: "hereafter all watchmen provided for by the United States Government for service in any of the public squares and reservations in the District of Columbia shall have and perform the same powers and duties of the Metropolitan police of said District" (emphasis added). 
There was some disagreement about whether this gave the watchmen police powers outside the parks. The D.C. Commissioners initially took the position that it did not. They offered to give the watchmen special appointments as "additional policemen" on the Metropolitan force so that they could act beyond park boundaries, but this was unsatisfactory to Public Buildings and Grounds officials. They did not want to allow the Commissioners an effective veto over the appointment of watchmen by making their full performance subject to the Commissioners' approval. The issue was referred to the Attorney General, who decided in July 1886 that the language of the law giving park watchmen "the same powers and duties of the Metropolitan police" could not mean that they had lesser jurisdiction.  The D.C. Commissioners accepted the opinion and conveyed it to their force. Henceforth the park watchmen carried the full authority of the Metropolitan Police without regard to location.