Cooper succeeded his deranged elder brother as county Member and as manager of the family interest, though he declined the colonelcy of the county militia.
Cooper voted against government ‘on all but the last’ division on the Scheldt inquiry, 30 Mar. 1810, according to the chief secretary, who by 24 Feb. had pressed for his attendance. This conduct was found curious: less than a month before, the lord lieutenant had informed him, ‘Mr Cooper supports us and must have the most [county patronage]’. Cooper was evidently in the government majority against the release of the radical Gale Jones, 16 Apr., and was listed ‘government’ by the Whigs at that time. On 1 June 1810 and subsequently he voted against Catholic relief. He voted against government for the repeal of the Irish newspaper advertisement tax in May 1811. On 24 Feb. 1812 he was in the government minority on the sinecure paymastership and on 21 May likewise on Stuart Wortley’s motion, which made (Sir) Henry Montgomery’s assertion soon afterwards that Cooper was one of the Irish Members who had no intention of going into opposition with the Liverpool administration seem somewhat unqualified. After the election of 1812 he was listed a ministerialist and reprimanded by the chief secretary for making difficulties about his attendance in February 1813.
In the session of 1812-13 Cooper prepared another bill to improve the Irish grand jury system which, after amendment in the Lords, was lost by Parnell’s motion, there being nobody ‘to take care of the bill’ while Cooper was detained in Ireland by ‘a nervous fever’. Cooper readily yielded the subject to William Fitzgerald, but as the latter was inactive he resumed the brief and, after being detained in Ireland by a bid to rescue an orphan accidentally enlisted into the army while under his care, he voted for inquiry into the state of Ireland, 26 Apr. 1816, and appeared on 6 May to move a bill to consolidate the Irish grand juries’ fund-raising activities. He floundered and Fitzgerald once more took it off his hands, pending a committee. The latter once more disappointed Cooper, who again moved his bill, but readily surrendered it before committal, 2 May 1817, and ‘could not see’ his way to revise the bill; though he was eloquent on the disadvantages of postponement, 14 May, when it was again placed in Fitzgerald’s care. Peel paid Cooper a compliment on the occasion and he was prepared to swallow Fitzgerald’s bill, 13 June 1817, though a speech of 29 Apr. 1818 suggested that he felt he had been cavalierly treated.
Cooper was in the opposition majority against the ducal grant, 15 Apr. 1818, and in the minority for the repeal of the Irish window tax on 21 Apr. He voted with government against Tierney’s censure motion, 18 May 1819, but his contribution to debate had by then virtually ceased. He died in 1830, having made way for his son that year.