Following his appointment in February 1866 as the director of the National Gallery, Boxall practically gave up painting. His directorship lasted eight years, during which he oversaw the construction of Edward Middleton Barry's celebrated eastern extension. In 1869 Boxall negotiated the purchase of Sir Robert Peel's collection of Flemish and Dutch paintings for £7,500. With this purchase the Dutch Golden Age became one of the strengths in the Gallery's holdings. Both of the Gallery's paintings by Michelangelo were bought by Boxall, The Entombment in 1868 and the Manchester Madonna in 1870. The authenticity of the former was called into question by the House of Lords in 1869, but is now generally regarded to be genuine – unlike another of Boxall's controversial acquisitions, the "Suermondt Rembrandt" 7, now attributed to Nicolaes Maes.