Lewis leaves next month and her duties, including overseeing a 50-strong team in five departments and a £10m comms and events budget, will fall to executive vice-president of people and comms Simon Cartwright.
The company was informing staff and retained consultants of her as PRWeek went to press.
Lewis, who is five months pregnant, said the timing of her departure coincided with a period of maternity leave and 'seemed like a good time to take a break'.
She said she has no firm professional plans for the future but is considering a number of non-executive roles, understood to include arts bodies.
Divisions to be overseen by Cartwright are; media relations, headed by Niamh Byrne; CSR and community relations, headed by Brigid Burnham; international PR and branding, headed by Catherine Bateman and corporate presentation, events and exhibitions, headed by Dominic Denny.
Sponsorship, which was under Lewis's remit, is to be moved to a separate global brand department. A new head of sponsorship is set to be announced next week.
Lewis has been with Orange for six years, joining as head of PR and sponsorship.
During this time she has overseen PR through four owners, from Hutchison Whampoa to Mannesman, to Vodafone and latterly France Telecom.
She said her departure is not set to affect the firm's relationship with its retained agencies, Edelman and Cohn & Wolfe on the corporate side, Citigate Dewe Rogerson covering a financial brief and MacLaurin on arts-related PR.