She takes over from Paris-based Linda Pavy, who will now run B-M's healthcare nutrition work.
Existing Onyx projects will be handled by associate director Jane Wrenn until October, when the agency will wind up. Clients are in the process of deciding whether to move to B-M.
Winterhalter previously worked as European director of healthcare at Weber Shandwick, before being made redundant in 2002.
She will assume her new role on 6 May, based in B-M's London office and reporting to CEO for Europe, Middle East and Africa Jeremy Galbraith.
'Burson-Marsteller is making a serious investment in the healthcare sector on a global basis and my focus will be to grow its pan-European network capabilities, as well as strengthen existing business in key markets across Europe,' said Winterhalter.
Onyx was set up to follow an ad agency structure, with a creative director and media planner under the two directors, calling on freelance support where necessary.
The agency's existing clients include Cephalon, Teva Pharma and Solvay. It also handled PR activity for the drug Xyrem, manufactured by UCB, which treats adult narcolepsy with cataplexy.
B-M's UK healthcare practice is headed by former Manning Selvage & Lee healthcare chief Jennifer Garratt, who joined the agency last year (PRWeek, 27 June 2007). Garratt will now report to Winterhalter.