The first building on the site was a Franciscan friary, constructed in about 1300, which was abandoned in 1533 after the Protestant Reformation. In 1636, George, Duke of Brunswick-Lüneburg, began construction of the castle on the site as his residence. In 1742 the north-west wing was renewed. Between 1816 and 1844, the architect Georg Ludwig Friedrich Laves fully re-built the castle. The column portico with six Corinthian columns was built during this period.
During World War II, the Leineschloss burnt out entirely after British aerial bombings. The architect Dieter Oesterlen re-built the castle between 1957 and 1962.