He matriculated at Christ Church, Oxford on 27 April 1703, aged 19 and was admitted at Inner Temple in 1709.
[2] Duncombe was returned unopposed as Member of Parliament for Downton in a by-election on 9 May 1711 in succession to his late uncle.
He is not recorded as having voted in any of the divisions during his time in Parliament but it is not possible to distinguish his contributions from those of other Duncombes.
At the 1734 British general election, he was returned as MP for Ripon by John Aislabie, whose daughter had married his brother-in-law, Sir Henry Slingsby, 5th Baronet.
He voted against the Administration, was not asked to stand at the 1741 British general election,[3] Duncombe died on 23 March 1746 leaving three sons and two daughters.