Commander of cavalry division of the army of the Sutlej under Lord Hugh Gough in the first Anglo-Sikh war was born on 1 February 1781, the son of John Thackwell. A veteran of Peninsula and Waterloo, he assumed command of the army of the Indus in the Afghan campaign of 1838-39. He also commanded the cavalry division of Sir Hugh Gough's army in the campaign against the Marathas of Gwalior at the close of 1843. In the first Anglo-Sikh war, he was in command of the cavalry at Sabhraon on 10 February 1846. In November 1846, he was promoted major-general.

In the second Anglo-Sikh. war, Major General Thackwell commanded the 2nd division of the infantry. On the death of Brigadier Cureton in the battle at Ramnagar, he took over the command of the Cavalry Division. Gough sent a force of eight thousand men under Thackwell to pass the river higher up, and help dislodge the Sikhs from their position by moving on to their left flank and rear. Thackwell crossed the river at Wazirabad and, on 3 December 1848, encamped near Sadullapur. He was attacked by the Sikhs, and the British pickets were driven out of three villages. Thackwell also saw action at Chelianvala and. Gujrat.

At Chelianvala, Thackwell's cavalry brigade under Pope courted disaster. Pope's brigade had advanced to protect the flank and movement of the 3rd Infantry Division under Major-General Campbell, when some hundred ghorcharhis fell upon them, and by successive onslaughts broke up the British cavalry line and cut down, their horsemen. The Sikh horsemen swept the field like lightning and their Khalsa war-cries so frightened the entire British cavalry brigade as if they had seen a ghost. They fled, galloping their own horse artillery and leaving behind their comrades at the mercy of the Sikhs. Dalhousie records the rout of Thackwell's 2nd cavalry brigade in these words : "The cavalry on the right disgraced their name and the colours they carry:... They galloped on into the Field Hospital, among the wounded and never stopped till they were brought up by the Chaplain, who was administering to the wounded and who, pistol in hand, declared he would shoot at the first man who passed him."

Thackwell wrote Narrative of the Second Sikh War in 1848-49, published in London in 1851. The chronicle is a detailed account of the battles of Ramnagar, Chelianvala and Gujrat.

In 1854, Thackwell was promoted lieutenant-general. He died on 8 April 1859.