Viner succeeds Alan Rusbridger, who has led the Guardian's editorial output for the past 20 years and is taking on the role of head of chair of The Scott Trust in 2016, the body responsible for safeguarding the Guardian's future.
The 44 year old is believed to have beaten Newsnight editor and a former deputy editor of the Guardian, Ian Katz, to the top job. She was the popular choice among staff, having won 53 per cent of the votes in an internal election organised by the paper’s National Union of Journalists chapel earlier this month.
She becomes the 12th editor in the newspaper's history, which has its roots back to the Manchester Guardian in 1821. In another first, Viner will also be the first state school-educated editor for the liberal news brand since Alfred Powell Wadsworth in 1956.
Liz Forgan, chair of The Scott Trust, said: "Katharine shone through in what was a tremendously strong line up of candidates.
"In her 18 years at the Guardian she has done almost every editorial job in the organisation, including running Guardian US and Guardian Australia, and has shown herself to be an inspiring and courageous leader.
"She has embraced the huge changes in the industry with creativity and relish whilst bringing with her a deep commitment to the Guardian's traditions of plural, liberal journalism.
"The Scott Trust is confident that Katharine, building on the extraordinary achievements of Alan Rusbridger, is the right person to lead the Guardian and the Observer as we expand on new platforms and in new markets".
Viner said: "Being editor-in-chief of the Guardian and Observer is an enormous privilege and responsibility, leading a first class team of journalists revered around the world for outstanding reporting, independent thinking, incisive analysis and digital innovation.
"I am honoured to succeed Alan Rusbridger, whose remarkable tenure will be remembered for prize-winning journalism and pioneering advances in the Guardian’s global reach.
"Building on his track record, I intend to lead a media organisation that is bold, challenging, open and engaging. It will be a home for the most ambitious journalism, ideas and events, setting the agenda and reaching out to readers all around the world."
Alan Rusbridger, the outgoing editor-in-chief, said: "Kath rose up through the Guardian as an inspired magazine and features editor.
"She took Australia by storm before heading up the Guardian’s American operation. She will bring immense experience, flair, warmth, imagination and formidable energy to her new role as editor of the Guardian."
The appointment of the new editor-in-chief was made following a search by The Scott Trust in conjunction with an executive search firm. There were 26 applications in total.
Commenting on the hiring process, Forgan added: "This has been a thorough, transparent and, for the first time, international process.
"We considered a very broad range of candidates across geographies, disciplines and backgrounds – including all those who took part in the editorial hustings – in our determination to leave no stone unturned in the search for the best person to lead this now global and much respected media organisation."
The editors of the Guardian
1. 1821-1844: John Edward Taylor (aged 29)
2. 1844-1848: Russell Scott Taylor (aged 18) with Jeremiah Garnett (aged 50)
3. 1848-1861: Jeremiah Garnett exclusively (aged 54)
4. 1861-1872: John Edward Taylor II (aged 30)
5. 1872-1929: Charles Prestwich Scott (aged 25)
6. 1929-1932: Edward Taylor Scott (aged 45)
7. 1932-1944: William Percival Crozier (aged 52)
8. 1944-1956: Alfred Powell Wadsworth (aged 52)
9. 1956-1975: Alastair Hetherington (aged 37)
10.1975-1995: Peter John Preston (aged 37)
11.1995-2015: Alan Charles Rusbridger (aged 41)
12. 2015: Katharine Viner (aged 44)