The first written mention of Kostomlaty is from 1223. Existence of a castle in Kostomlaty was documented in the mid-14th century, but in 1553 at the latest, the castle was already abandoned. In the 1420s, the village and the castle were acquired by
Hynek Boček of Poděbrady, and in 1458, Kostomlaty passed into ownership of King
George of Poděbrady. Later it was inherited by his descendants, then it often changed owners. Among the most important owners was the Sporck family, who held it from 1647 to 1722 as a part of the
Lysá estate. ==Demographics==