He completed his education at Trinity College, Dublin, and, apparently without waiting to take a degree, entered the Foreign Office on 15 October 1817.
In this negotiation, though he did not carry the British points, he obtained the approval of his government, which signed the Oregon Treaty in 1846.
Pakenham left Washington on a leave of absence in May 1847, and, after remaining in Europe for an unusually prolonged period, ultimately preferred to retire on a pension rather than return.
However, on 7 August, he was sent back to Lisbon on a special mission to congratulate King Pedro V of Portugal on attaining his majority.
He returned to England once more in October 1855, was awarded a diplomatic pension of the second class, and retired to Coolure, Castlepollard, where he died, unmarried in 1868.