Upon the outbreak of the Crimean War, Chapman was attached to the 1st Division, commanded by Prince George, Duke of Cambridge, as senior engineer officer. He took part in the Battle of the Alma and was mentioned in dispatches of 28 September 1854, taking command of engineering operations during the Siege of Sevastopol. He was present at the Battle of Inkerman and was mentioned in dispatches on 11 November 1854, 23 June 1855 and 9 September 1855. On 5 July 1855 he was made a Companion of the Order of the Bath and received honours from several other foreign governments, including being made an officer third class of the Order of the Medjidie. He was awarded a pension by the British government for distinguished service on 23 November 1858. On 8 April 1856 Chapman was appointed as the Chief Royal Engineer in the London District, before transferring to Aldershot in September 1857. On 1 April 1859 he was promoted to regimental lieutenant-colonel. From 1 September 1860 he served as deputy adjutant-general of Royal Engineers at Horse Guards for five years. He was made a Knight Commander of the Order of the Bath on 13 March 1867 and on 8 April he was appointed governor and commander-in-chief of Bermudas. On 1 July 1870 he resigned this position and accepted the appointment of Inspector-General of Fortifications and Director of Works at the War Office.