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. General John Whitelocke had arrived to take command of the combined forces, and as Whittingham's staff appointment ceased on the amalgamation of the forces, Whitelocke made him an extra aide-de-camp to himself. He took part in the disastrous attack on Buenos Aires and in the capitulation on 6 July, and sailed for England on 30 July. He gave evidence before the general court-martial, by which Whitelocke was tried in London in February and March 1808. Owing to his having served on Whitelocke's personal staff, Whittingham's position was a delicate one; but he acquitted himself with discretion.