Reid, who was group client partner at Carat and had spent 12 years at the Dentsu Aegis agency, will manage £600m worth of client billings alongside Danny Donovan, the other joint managing director of MediaCom London.
She replaces Luke Bozeat, who was promoted to chief operations officer at Britain's biggest media agency.
MediaCom credited Reid as being "instrumental in helping Carat retain the Diageo account" in the UK as part of a global review earlier this year.
Reid began her career as a media planner at Universal McCann, initially at Michaelides & Bednash, and moved to Carat in 2004 where she was head of strategy, before taking a commercial role managing £200m of billings for UK and global clients. She has also been a judge at the 2016 Campaign Media Awards.
Carat UK has six other client partners and expects to replace Reid shortly.
MediaCom UK chief executive Josh Krichefski said: "She is an inspirational leader and I am delighted to have somebody of her ability representing MediaCom in the industry. Her strategic credentials and passion for modern working culture make her a perfect fit for MediaCom."
MediaCom was hit by the loss of Volkswagen in June but has largely offset that by winning Paddy Power Betfair’s consolidated media account and British Gas.