Lyall, who was previously the managing partner for digital, will be responsible for the agency's planning offering and will report to Mindshare's managing director, Ita Murphy.
She will lead Mindshare's new "Open Source Planning" approach, working closely with the client leadership and media planning teams across a range of clients including Volvo, Unilever and Ford.
Lyall's promotion follows the appointment of Mark Creighton, the former managing director of i-level, to the role of chief operating officer last August.
Murphy said: "Over the years, Jo has done a terrific job of building our digital expertise; developing invention and latterly building our social marketing expertise.
"This newly created role reflects the broader influence that Jo will have across our clients' business."