Haygarth joins Andy Fowler, the founder and executive creative director, Kevin Brown, the digital director, and Steve Shannon, the creative director, on the management team as a fellow partner.
Haygarth will be responsible for extending the agency's client list, overseeing existing accounts, such as Sky and The Sun, and the day-to-day running of the business.
Before joining The Hospital Club, Haygarth worked both client- and agency-side.
She started her career in marketing in 1994 at the Samaritans, before moving to the British Red Cross and Barnardo's.
In 2000, she joined HHCL as an account director, later joining TBWA\ and Rainey Kelly Campbell Roalfe/Y&R as a group account director. She has been in her current role for the past two years.
Fowler said: "Juliet's arrival comes at an exciting time for us, and she will be pivotal in managing the continued long-term growth of the agency."