Millar also wrote many catalogues and other works. He published a book on Thomas Gainsborough in 1949, and another on William Dobson in 1951. He co-wrote English Art, 1625–1714 with Margaret Whinney in 1957. The same year, he selected 37 works by van Dyck to be exhibited at the winter exhibition of Flemish art at the Royal Academy. He published a catalogue of the Tudor, Stuart and Early Georgian paintings in the Royal Collection in 1963, followed by a volume of Later Georgian pictures in 1969 and a two volume catalogue of Victorian pictures in the Royal Collection in 1992. He edited the Inventories and Valuations of the King's Goods, published in 1972, based on an inventory of the possessions of Charles I originally compiled in around 1639 by the first Surveyor, Abraham van der Doort. Millar reckoned that Charles' collection was the best single English collection of paintings ever made, but it was broken up and sold at auction in 1649 after the king was executed. Also in 1972, Millar also wrote the catalogue for an exhibition of "The Age of Charles I" at the Tate Gallery. After 23 years as Deputy, Millar succeeded Anthony Blunt as Surveyor in 1972, and was advanced to KCVO in 1973. The Royal Collection became more professional in its outlook under his stewardship. The Surveyor had historically been a part-time position, under the overall supervision of the Lord Chamberlain since 1625, based at St James's Palace. Millar was the first full-time Surveyor, and the Royal Collection employed its first full-time picture restorer in 1981. He broke free from the Lord Chamberlain in the 1980s, when he became the first Director of the Royal Collection in 1987, adding overall responsibility for the Surveyor of the Queen's Works of Art and the Royal Librarian. Millar continued to publish catalogues and other works. He published The Queen's Pictures in 1977, a general account of the Royal Collection, and wrote the catalogues for exhibitions of works at the National Portrait Gallery by Sir Peter Lely in 1978 and "Van Dyck in England" in 1982, selecting himself the 65 paintings and 22 drawings for the latter exhibition. He also wrote for The Burlington Magazine and other art history journals. Between 1962 and his retirement in 1988, he was responsible for mounting 12 public exhibitions at the Queen's Gallery at Buckingham Palace, attracting almost 2.5 million visitors in all.