She joins as the chief marketing officer with a brief to help reinvigorate the agency, which has been beset by internal upheaval and a series of account losses.
Mitchem, 39, who was the group marketing director at M&C Saatchi, joins the five-strong management committee that will run Lowe.
The new-business role at the agency has been vacant since Matt Edwards resigned as the new-business director in January to become the new-business and marketing director at WCRS.
Mitchem said: "M&C Saatchi is like a family. But this is a much bigger job with the opportunity to work on a fantastic brand, which Garry is determined to return to its former glory."
Meanwhile, Lace has returned to his former agency, Grey London, to hire Tina Fegent, its commercial director. Lace hired Fegent for Grey from Orange, where she was the purchasing manager for sales and marketing.