McCartney will quit Good Relations in November after six years at the Bell Pottinger agency, having previously been head of business liaison for the Britain in Europe campaign.
Sixteen-strong Champollion, which specialises in issues management and media handling, is looking to strengthen its lobbying firepower.
MD Simon Buckby said: ‘Champollion has been lucky enough to more than double in size this year and we want to continue our expansion.
'By appointing Estelle we will be adding an experienced and skilled lobbying offer to our range of campaigning and media services. We are sure she will help us maintain an ethical dimension and high standards in all our work for our clients in Westminster and Whitehall.’