The newspaper named Viner on Friday as the 12th editor in its 194-year history. The first woman to hold the post, she replaces Alan Rusbridger, who is stepping down after 20 years.
Under Rusbridger, the liberal London-based newspaper expanded its digital presence and international impact, including online editions in the U.S. and Australia.
It was instrumental in publishing documents obtained by the anti-secrecy site WikiLeaks and last year shared a Pulitzer Prize with the Washington Post for stories based on files leaked by ex-National Security Agency contractor Edward Snowden.
The scoop brought the paper into conflict with British intelligence services, but Rusbridger said it had prompted a debate about the extent of poorly regulated snooping.