She will report to Denise Kaufmann, partner and managing director of client service, and Stephen Waddington, European digital and social media director.
Ketchum has not had a digital head since the departure of Mo Lishomwa in September to join Adobe, which was followed by the arrival of Waddington.
Guthkelch has been in her current role for two years, helping to modernize the postal service’s 58-person comms team and overseeing the development and execution of internal and external comms programmes.
Her previous experience includes roles as account director at online agency RE Systems and joint investor relations and digital comms handler at property company Land Securities.
Kaufmann said: ‘Abby combines first rate social and digital communication expertise with the knowledge and frontline experience of helping brands modernise their communications.’