Secretary to the Treasury through whom the business of the Isle of Man passed in the years after the Revestment.

The correspondence shows the shape of the office. In June 1766 he wrote to William Burke at Secretary Conway's office conveying the Lords of the Treasury's direction that the Governor of the Isle of Man be allowed a particular expense. Governor Wood's despatches were minuted as to Grey Cooper and read at the Treasury in October of that year. In January 1767 William Fraser sent him a copy of Wood's letter of 5 December 1766 to be laid before the Lords of the Treasury for their information. He was still the address for Manx business a decade later: Governor Smith's letter of January 1779 asking for a new copper coinage for the Island, and reporting on the cambric manufactory lately set up there at government expense, is directed to Sir Grey Cooper.

In the Commons he was a Treasury man of the sort who tells at divisions and carries the detail. He was named with Lord Beauchamp as the mover of the Grenada, Tobacco and Cotton Acts, whose object was the augmentation of the revenue. He argued for the renewal of the Bank's charter on the ground that the question was not whether two millions was an adequate price for the Bank's profits but whether it was an equivalent for the charter. He opposed the East India Declaratory Bill of 1788 on its principle, closely enough that Pitt read out the clauses he had relied on and allowed that he had argued it fairly. The secret service accounts of the 1770s show large sums issued to Cooper and to John Robinson as secretaries of the Treasury, a fact the opposition made what it could of.