Cooper continued to sit for county Sligo on the family interest, which he had managed since 1806 following the derangement of his elder brother Joshua Edward Cooper†. At the 1820 general election he was again returned unopposed. A mostly silent Member who ‘attended occasionally’, when present he gave general support to the Liverpool ministry, by whom he was listed as having secured a collectorship of excise for Major Charles King O’Hara, eldest son of his colleague.
At the 1826 general election Cooper, a ‘veteran in the cause of the constitution as established in 1688’, was re-elected unopposed.
At the 1830 dissolution Cooper, ‘feeling the duties’ of Parliament ‘too arduous’ and citing his poor health, retired in favour of his eldest son Edward Joshua. Rumours that he was secretly campaigning against his former colleague, who had gone into opposition, were dismissed by the Sligo Journal. He died on the second day of the contest, following which his son was returned in absentia with his chair ‘dressed in the deepest mourning’. A local obituary paid tribute to his ‘spotless integrity’ and ‘munificent charities’, eulogizing him as ‘strictly the Protestant and the Christian’.
