Transferred with his regiment to the Peninsular War, he was present at the Battle of Albuera in 1811, where he commanded the British light infantry. After the battle he temporarily commanded a brigade of Fusilier regiments. Later in the year he suffered a leg wound and returned to Britain on sick leave.
Early in 1812, he was sent to Canada as Inspecting Field Officer of Militia. After the outbreak of war with America, Pearson was appointed Commandant of Fort Wellington at Prescott, in the middle of 1813. Late in the year, an American expedition under James Wilkinson passed by Prescott. Pearson attached part of his garrison to the pursuing British force under Lieutenant Colonel Morrison and led them at the decisive Battle of Crysler's Farm.
After the end of the War, Pearson returned to command his regiment. He was knighted and promoted to Major-General in 1833 and to Lieutenant General in 1841. He died in 1847.
- Gray, Randal. "Pearson, Sir Thomas". Oxford Dictionary of National Biography (online ed.). Oxford University Press. doi:10.1093/ref:odnb/53724. (Subscription or UK public library membership required.)
- Donald E. Graves (September–October 2011). "Forgotten Hero in a Forgotten War". Humanities (magazine).
- Graves. Donald, Fix Bayonets! A Royal Welch Fusilier at War Toronto: Robin Brass Studio, 2006. ISBN 978-1-896941-27-1.
- Painting by unknown artist of "Lieutenant General Sir Thomas Pearson, KCB, KCH" at BBC "Your Paintings" website
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