Henderson will run Initiative's £110m Tesco account, while Harwood will have responsibility for Initiative clients such as Burger King, Match.com, Quorn and Sonos.
Both Henderson and Harwood will report to Anna Watkins, managing director of Initiative.
As client director at Mindshare from 2007 until last month, Henderson worked on clients such as Argos, Homebase, Zurich, IHG, Brewin Dolphin and Everest in what was his second stint at the agency.
Henderson was a retail media strategist between 2005 and 2007 at Tesco’s loyalty firm Dunnhumby. During Henderson’s first stretch at Mindshare he worked on clients such as Abbey (now Santander), Homebase, Warburtons and Ford.
He started his career in airtime sales at ITV sales house Carlton TV between 1996 and 2000.
Harwood was responsible for a range of businesses including AXA, Peugeot, Sony, Intel, Bel UK, Bosch Group, Waterstones and Hasbro during her 12 years at OMD UK.
Prior to joining OMD UK in 2000, Harwood was a planner and then planning director at CDP working on brands such as Honda, MGM Home Entertainment, Irish Ferries, ANA.
Harwood started her career, as a planner buyer in the media department of JWT.
Alex Altman, chief executive of Initiative UK, said: "These are two really strong appointments for Initiative as both Steve and Tania have a fantastic heritage in media and bring vast knowledge and experience to their respective roles."
Kennett is understood to have left Initiative to taking a career break and go travelling, while Pattison is not thought to have a job to go to.Follow @MaisieMcCabe