On July 1, 1870 (nearly 100 years after the founding of the country) the United States finally got an official Justice Department.
Back in the days of yore, when America was just an infant discovering how awesome its hands and feet were and such, justice lay in the hands of the Attorney General, who was just a part-timer. The role grew along with the country, and for a while the AG was responsible for giving legal advice to both the U.S. Congress and the President, which is a hell of a client list to keep up with. By 1819, that part of the job was dropped.
In the late 1860s, Congressman William Lawrence conducted an inquiry into the creation of a whole department headed by the AG and composed of various department solicitors and US attorneys. Lawrence introduced a bill in Congress to create such a department on February 19, 1868, but then he got distracted by the (unsuccessful) second attempt to impeach President Andrew Johnson. Almost exactly two years later, a second bill was introduced by Rhode Island Representative Thomas Jenckes. The bill passed both the house and the senate and was signed into law on June 22, 1870. The DOJ opened for business on July 1.