On July 27, 1789, Congress created the State Department, which became an important part of the Executive Branch established under the new Constitution. Since then the Department of State has grown significantly over the years. The first Secretary of State, Thomas Jefferson (1790-1793), oversaw a small staff of one chief clerk, three other clerks, a translator, and a messenger and only maintained two diplomatic posts, in London and Paris, as well as 10 consular posts.
World War I (1914–1918) and World War II (1939–1945) brought vastly increased global responsibilities to the United States as it emerged as a preeminent power. New challenges after the end of the Cold War and the fall of the Soviet Union, included:
the newly independent states,
the global economy,
the security of the American overseas presence.
To address these changing global circumstances, the number of domestic and overseas employees (not counting local employees) grew to:
1,228 in 1900,
1,968 in 1940,
13,294 in 1960,
and 15,751 in the year 2000.
The number of diplomatic posts increased from 41 in 1900 to 168 in 2004 and continues to grow.
You can read more on the history of the Department of State on the Office of the Historian website.