McGarahan led comms around the 1 January deal which saw CAFC Holdings Limited, a Swiss investment vehicle led by ex-Newcastle United vice-president Tony Jimenez and new chairman Michael Slater, assume control of the League One club.
McGarahan will advise the Charlton board on strategic and corporate PR and high-profile announcements.
He supported the former Premier League club’s media relations around the sacking of manager Phil Parkinson last week.
Most recently McGarahan helped overhaul the global PR structure of Electrocomponents, the largest electronic and industrial distributor.
He has also worked as a consultant for waste management group Shanks and previously held in-house roles at Equitable Life, AXA and leading PR surrounding the nationalisation of Bradford & Bingley.