 In May 1667, in the war's final year, Downing was made secretary to the commissioners of the treasury, his appointment being much welcomed by Pepys, and he took part in the management and reform of the Treasury.He was appointed a commissioner of the customs in 1671.The same year he was again sent to Holland to replace Sir William Temple, to break up the policy of the Triple Alliance and incite another war between the Dutch Republic and England in furtherance of the French policy.