Macdonald is to take the newly created role of strategic relations director reporting to Capita's policy and public affairs director John Tizard.
She said the post would have more of a pure lobbying focus than her role at BT Government, where she was head of government relations and communications.
'After three years with BT, it's time for me to move on', she said, adding that she wanted to 'concentrate purely on the political lobbying aspect of my career which I enjoy so much'.
Macdonald, on gardening leave from BT Government, a competitor for government contracts with Capita, added that her new role will involve working closely with chairman Rod Aldridge.
She joins at a time when Capita is seeking to further capitalise on its transport work. It already runs the congestion charge in London and is seeking further opportunities in the sector. It is estimated that 35 cities in the UK are considering the possibility of congestion charging schemes of their own.
It emerged last month that Capita will regain the £27m it has spent on setting up the scheme in London, even if it fails. Assurances are understood to have been given that Transport for London will reimburse the amount even if the scheme should fail though errors by Capita.
A spokesman for Fishburn Hedges, which works for both Capita and Transport for London, insisted that the scheme had already shown itself to be a success.
FTSE 100 company Capita is one of the UK's major players in the public sector outsourcing market. Last month it announced pre-tax profits of £98.2m in 2002, a 36 per cent rise on the previous year.
A BT spokesperson said that BT Government was planing to reorganise its government relations following Macdonald's departure.
A decision on whether or not to replace her is pending the outcome of this reorganisation.