Who was the governor of madras in 1819 to 1826?

That would be Major-General Sir Thomas Munro. He was a Scottish soldier and a significant colonial administrator.

His official term as Governor of Madras began in June 1820. It ran right through to his death in office in 1827. So, he's definitely your man for that period. Munro is best remembered for establishing the Ryotwari System. This was a major land revenue reform that dealt directly with the peasant cultivators. He remains a very influential figure in the history of southern India, for sure.