In 1872 the Duke of Argyll, then Secretary of State for India, nominated Mallet to the Council of India. In 1874 he was appointed Permanent Under-Secretary of State for India, a position left vacant by the death of his first cousin Herman Merivale. He remained in that post until his retirement in 1883, touring India in 1875–76, which was facilitated by the opening of the Suez Canal in 1869. Mallet served on a royal commission on the laws relating to copyright in 1876, and was a commissioner for the British representation at the Paris exhibition of 1878. In the same year he and Lord Reay represented India at a Monetary Conference in Paris, convened as a result of a fall in the price of silver relative to gold. Mallet retired in 1883 but was recalled in 1887 to serve for a short period on a Royal Commission on Precious Metals.