In 1952, Castle Howard was opened to the public by its then-owner, Lord Howard of Henderskelfe, a younger son of Geoffrey Howard. It is now owned by a Howard family company, Castle Howard Estate Limited, and managed by the Hon. Nicholas Howard (the second son of Lord Howard of Henderskelfe) and his wife, Victoria. The 3rd Earl of Carlisle first spoke to William Talman, a leading architect, but commissioned Vanbrugh, a fellow member of the Kit-Cat Club, to design the building. Castle Howard was that gentleman-dilettante's first foray into architecture, but he was assisted by Nicholas Hawksmoor. Vanbrugh's design evolved into a Baroque structure with two symmetrical wings projecting to either side of a north–south axis. The crowning central dome was added to the design at a late stage, after building had begun. Construction began at the east end, with the East Wing constructed from 1701 to 1703, the east end of the Garden Front from 1701 to 1706, the Central Block (including dome) from 1703 to 1706, and the west end of the Garden Front from 1707 to 1709. All are exuberantly decorated in Baroque style, with coronets, cherubs, urns and cyphers, with Roman Doric pilasters on the north front and Corinthian on the south. Many interiors were decorated by Giovanni Antonio Pellegrini.

The Earl then turned his energies to the surrounding garden and grounds. Although the complete design is shown in the third volume of Colen Campbell's Vitruvius Britannicus, published in 1725, the West Wing was not yet started when Vanbrugh died in 1726, despite his remonstration with the Earl. The house remained incomplete on the death of the 3rd Earl in 1738, but the remaining construction finally started at the direction of the 4th Earl. Vanbrugh's design was not completed: the West Wing was built in a contrasting Palladian style to a design by the 3rd Earl's son-in-law, Sir Thomas Robinson. The new wing remained incomplete, with no first floor or roof, at the death of the 4th Earl in 1758. Although a roof had been added, the interior remained undecorated by the death of Robinson in 1777. Rooms were completed stage by stage over the following decades, but the whole was not completed until 1811 under Charles Heathcote Tatham.

A large part of the house was destroyed by a fire on 9 November 1940. The dome, the central hall, the dining room and the staterooms on the east side were entirely destroyed. Antonio Pellegrini's ceiling decoration, the Fall of Phaeton, was lost when the dome collapsed. In total, twenty pictures (including two Tintorettos) and several valuable mirrors were lost. The fire took the Malton and York Fire Brigades eight hours to bring under control. Some of the devastated rooms have been restored over the following decades. In 1960–61 the dome was rebuilt, and in the following couple of years Pellegrini's Fall of Phaeton was recreated on the underside of the dome. The East Wing remains a shell, although it has been re-roofed. In 2009 an underwater ground-source heat recovery system was installed under the castle's lake that halved the heating bill. According to figures released by the Association of Leading Visitor Attractions, over 269,000 people visited Castle Howard in 2019.

Castle Howard has extensive and diverse gardens. There is a large formal garden immediately behind (i.e., on the south side of) the house. The house is prominently situated on a ridge and this was exploited in the development of an English landscape park, which adjoins and opens out from the formal garden. The gardens are Grade I listed on the National Register of Historic Parks and Gardens. Two major garden buildings are set in this landscape: the Temple of the Four Winds at the end of the garden, and the Mausoleum in the park. There is a lake on either side of the house. There is a woodland garden, Ray Wood (formerly Wray Wood), immediately east of the house, and a walled garden which contains decorative rose and flower gardens. The Ray Wood walls date from the 18th century and were restored in 2007. Further buildings outside the preserved gardens include Nicholas Hawksmoor's Pyramid, restored in 2015, an obelisk, and several follies and eyecatchers in the form of fortifications which have been restored in recent years; these include Carrmire Gate and Pyramid Gate. In nearby Pretty Wood, there are two more monuments, The Four Faces and a smaller pyramid by Hawksmoor.