Military career
He served with his regiment during the hard-fought war against Hyder Ali (1780–1783), under his namesake Major Sir Hector Munro, 8th of Novar. Thomas also later served alongside another namesake John Munro, 9th of Teaninich. Thomas served again with his regiment in the first campaign against Tipu Sultan (1790–1792). He was then chosen as one of four military officers to administer part of the territory captured from Tipu, where he remained for seven years learning the principles of revenue survey and assessment which he afterwards applied throughout the presidency of Madras. After the final downfall of Tipu in 1799, he spent a short time restoring order in Kanara; and then for another seven years (1800–1807) he was placed in charge of the Ceded Districts ceded by the Nizam of Hyderabad, where he introduced the ryotwari system of land revenue. After a long furlough in Britain, during which he gave valuable evidence upon matters connected with the renewal of the East India Company's charter, he returned to Madras in 1814 with special instructions to reform the judicial and police systems. In response to a question put to him in 1813, concerning the renewal of the charter, he stated:
I do not exactly know what is meant by civilizing the people of India. In the theory and practice of good government they may be deficient; but if a good system of agriculture, if unrivalled manufactures—if a capacity to produce what luxury or convenience demands—if the establishment of schools for reading and writing—if the general practice of kindness and hospitality—and, above all, if a scrupulous respect and delicacy towards the female sex, are amongst the points that denote civilized people, then the Hindoos are not inferior in civilization to the people of Europe.
He was affectionately called 'mandrolayya' by local Telugu people after he brokered peace between local Polygars.
On the outbreak of the Pindari War in 1817, he was appointed as brigadier-general to command the reserve division formed to reduce the southern territories of the Peshwa. Of his services on this occasion Lord Canning said in the House of Commons:
He went into the field with not more than five or six hundred men, of whom a very small proportion were Europeans .... Nine forts were surrendered to him or taken by assault on his way; and at the end of a silent and scarcely observed progress he emerged... leaving everything secure and tranquil behind him.
In 1819, Munro was appointed a Knight Commander of the Order of the Bath (KCB).