He was determined to become a soldier, but unwilling to oppose his father's wishes during his lifetime, he temporarily entered the mercantile house of his brother-in-law, travelling for it in Spain.

On his father's death, on 12 September 1801 (aged 60), at Earl's Mead, Bristol, Samford, who was in Spain, became independent, and took steps to enter the army.

[1] On 12 November 1806 Whittingham sailed from Portsmouth as deputy-assistant quartermaster-general of the force, under Brigadier-general Robert Craufurd, intended for Lima; but on arrival at the Cape of Good Hope on 15 March 1807 its destination was changed, and on 13 June it reached Montevideo, recently captured by Sir Samuel Auchmuty.

On arrival at Gibraltar, however, he acted temporarily as assistant military secretary to Lieutenant-general Sir Hew Dalrymple, the governor, and, hearing of a projected campaign of the Spaniards under Don Xavier Castaños against the French, obtained leave to join Castaños as a volunteer, with instructions to report in detail to Dalrymple on the progress of affairs.

This special duty was approved from home on 2 July 1808, and on the 18th of the same month Whittingham was appointed a deputy-assistant quartermaster-general to the force under Sir Arthur Wellesley, but was ordered to remain with Castaños.

He reported constantly throughout these campaigns to the British minister in Spain, John Hookham Frere, as to the state and operations of the Spanish army.

[3] A short time previous to Wellesley's advance into Spain Whittingham joined the British headquarters on the frontier of Portugal, and became the medium of communication with the Spanish general Cuesta.

He went to Seville to recover, and lived with the British minister, Lord Wellesley; employing himself during his convalescence in translating Dundas's Cavalry Movements into Spanish.

At Isla-de-Leon, whither he went by Sir Arthur Wellesley's direction to see General Venegas about the defence of Cadiz, he was given the command of the Spanish cavalry, which he remodelled upon British lines.

[3] On 24 July 1812 the Majorca division embarked for the eastern coast of Spain to co-operate with the troops under Lord William Bentinck from Sicily.

On 4 June Wellington wrote from Madrid to the Duke of York, in anticipation of Whittingham's return home: "He has served most zealously and gallantly from the commencement of the war in the peninsula, and I have had every reason to be satisfied with his conduct in every situation in which he has been placed".

On Napoleon's escape from Elba Whittingham returned to Spain, at the special request of King Ferdinand, who conferred upon him the grand cross of the order of San Fernando.

Sir Henry Wellesley wrote at this time to Lord Castlereagh, expressing the sense he entertained of Whittingham's services both during the war and after, and reporting that he left Spain with the testimony of all ranks in his favour, "but without any other reward from the government for the valuable services rendered by him to the Spanish cause than that of being allowed to retain his rank in the Spanish army".

On his departure to take up the appointment, dated 5 October 1821, of quartermaster-general of the king's troops in India, the inhabitants presented him with the grand cross of San Fernando set in diamonds, while the non-resident proprietors of estates in the island gave him a sword of honour.

He was made a knight commander of the order of the Bath, military division, on 26 December, for his services at Bhartpur, and received the thanks of the House of Commons.