MacKay was press secretary to former Tory leader William Hague from 1997-99 and advised John Major during the then PM's leadership election in 1995. He also worked at Hill & Knowlton and joined Media Strategy in 1999.
Media Strategy MD Charles Lewington said: 'We have lost a good friend as well as a great professional. He was a natural at this job, building a really strong team. It's our responsibility to take forward that brilliant legacy.'
Friends are invited to a memorial in Edinburgh at Cramond Kirk on Saturday 10 December at 12.30pm – no black ties at his request, family flowers only.
Donations in memory of Gregor are welcome at Guy's and St Thomas' charity Lymphoma Fund (878), The Courting House, Guy's Hospital, St Thomas Street, London SE1 9RT.