In May 1855, he left for the Crimean War, in which he served firstly with his battalion, then from July 1855 as aide-de-camp to the commander of the 2nd Division, Lieutenant-General Markham, and finally from November 1855 as deputy assistant quartermaster general on the staff at Headquarters, being promoted brevet Major. He was mentioned in despatches and received the fifth class of the Turkish Order of the Medjidie and the British, Turkish and Sardinian Crimean medals.
In 1857, he was promoted Captain and Lieutenant-Colonel, and transferred, as a Lieutenant-Colonel, to the 95th (Derbyshire) Regiment in 1858, serving with that regiment at the end of the Indian Mutiny, for which he was again mentioned in despatches. From 1861 to 1862 he served as deputy adjutant general to the forces in Bombay, and was promoted brevet Colonel in 1863. He served, again as deputy adjutant general, in the Magdala campaign, for which he was awarded the CB and made an aide-de-camp to the Queen in 1868. He was adjutant general in the East Indies from 1869 to 1874.
He returned to England in 1874 as colonel on the staff, commanding the forces at Shorncliffe, and was appointed to command a brigade at Aldershot, with the temporary rank of Brigadier-General, in 1877.
He was promoted Major-General in March 1877. In February 1878 he was appointed to command the forces in South Africa, with the local rank of Lieutenant-General, and in October succeeded his father as 2nd Baron Chelmsford. He brought the Ninth Cape Frontier War to an end in July 1878, and was made a KCB in November.
In January 1879 he invaded Zululand (see Anglo-Zulu War), but the centre column of his forces was defeated at the Battle of Isandlwana. Because an invasion of Natal seemed likely as a result, Chelmsford was relieved of his command. However he defeated the Zulus at the Battle of Ulundi just before the arrival of his replacement, Sir Garnet Wolseley, which effectively ended the campaign. He left for England in July 1879, but Wolseley ensured in his despatches that Chelmsford receive all the credit for Ulundi, and he was awarded the GCB in August.
Lord Chelmsford became Lieutenant-General in 1882, Lieutenant of the Tower of London from 1884 until 1889, colonel of the 4th (West London) Rifle Volunteer Corps in 1887, full General in 1888, and colonel of the Derbyshire Regiment in 1889. He exchanged the colonelcy of the Derbyshires for that of the 2nd Life Guards in 1900, and was made GCVO in 1902.
He was the inaugural Governor and Commandant of the Church Lads' Brigade, a post he held until he died.