Reporting to MD Tanya Rose, Fox will work with the London agency's luxury travel clients such as Hotels Constance, the Saxon in Johannesburg and The Stein Group. She will also try to bring in new business.
Fox, who prior to Halpern was MD of agency Talk Luxury, starts in the role on 2 October.
Her appointment at Mason Rose follows Lucie Moore's relocation to the south of France this month.
At Halpern, Fox worked as a divisional director specialising in travel and lifestyle clients. She joined the agency, whose clients include Center Parcs, Virgin Limited Edition and Bridgehouse Hotels, in October 2003.