General Sir Thomas Westropp McMahon, 3rd Baronet, CB (14 February 1813 – 23 January 1892) was a senior British Army officer.
The younger Sir Michael Roberts Westropp, Chief Justice of the High Court of Bombay, was his cousin.
He went with the Lancers to India, under the command of Sir James Hope Grant, where he took part in the Sutlej campaign, seeing action at the Battle of Sobraon in 1846.
He then served in Turkey and the Crimea as assistant quartermaster-general of the cavalry division and was present at the battles of the Alma, Bokklava (with the Heavy Brigade), the Tchettaya, and siege of Sebastopol.
While in the Crimea, on the promotion of Sir James Yorke Scarlett, he was made lieutenant-colonel of the 5th Dragoon Guards in 1854, and commanded that regiment until he went on half-pay in 1861.