BPPA MD Peter Bingle said MacDonald has joined the agency as director and has been hired for her experience of Scottish politics and media.
Bingle would not divulge which clients MacDonald would be working for, but it is understood that she is set to assist with work for US fast-food giant McDonald’s and defence and aerospace company BAE Systems.
MacDonald relinquished her role as special adviser to the Scottish Secretary after Liddell’s departure from the Scottish Office following June’s Cabinet reshuffle.
Prior to this, she worked at Scottish Media Group as director of public affairs for its TV division and as network controller for factual programmes.
BPPA has also appointed Brunswick associate partner Fiona Mason as associate director. Mason is said to specialise in competition issues.
The responsibilities of the Secretary of State for Scotland were merged with those of the transport ministry following the June reshuffle.
MacDonald has effectively been replaced by former MSP Iain Grey, who is special adviser on Scottish matters to Scottish and Transport Secretary Alistair Darling.