Law, who is moving from Lowe London where she was the global business director on Unilever, will work on the agency's RBS Group business.
Harrison, who joins M&C Saatchi from Cheethambell JWT where he ran the agency's retail business, will run the Dixons Stores Group International account, which includes Currys, Currys Digital and PC World.
Before Cheethambell, Harrison worked at its sister agency JWT, where he was the group account director on Vodafone, B&Q and HSS.
The agency has also hired Anna Donaghey as a senior planner. She will work on the Transport for London and Lucozade business. Donaghey also moves from Lowe, where she was a planner working across the Unilever account.
Tim Duffy, the chief executive of M&C Saatchi, said: "2006 has been a year of change and growth at the agency. We're thrilled that Gillian, Mark and Anna are bringing their talent and experience to M&C Saatchi at such an exciting time."