She replaces Edward Felsenthal, who will continue as editor-in-chief of Time magazine and take on a new role as executive chairman.
Felsenthal has been editor-in-chief since 2017 and chief executive since 2018 when Time was taken over by tech entrepreneur Marc Benioff and his wife, Lynne.
Before becoming chief operating officer of Forbes, Sibley was chief revenue officer, chief sales officer and senior vice president of US and Europe. She has also held senior roles at The Wall Street Journal, Bloomberg and Conde Nast. She starts her new role on 21 November.
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