Rainey will oversee the agency as it aims to increase its national profile. She will also mentor Tarek Nseir, the agency's managing director, as the agency expands its offering.
The agency was founded by Nseir in 2001. Clients include Northern Rock, Bonhams and the NHS. Th_nk was recently valued at £25 million and employs 60 people.
Rainey said: "Th_nk combines the energy of a hungry growing business with the processes of a mature shop. I've also been impressed with Tarek's combination of dazzling technological flair with real commercial acumen."
Nseir said: "Having MT as our chairman is a major privilege for us in this challenging phase. Her track record of steering an agency through fast growth, coupled with her experience of working with some of the leading brands, is already proving invaluable."