Washington D.C. was not always the capital of the United States. The first capital was in New York City, in 1789. George Washington took his first oath as president on the balcony at Federal Hall. However, President Washington had to decide on a permanent location for the capital. Part of a compromise, he decided that it would be located in Philadelphia, Pennsylvania for 10 years, then moved to a location on the Potomac River.
Congress met in Washington D.C. for the first time in December 1800, which has now remained the capitol of the United States of America for more than 200 years.