Kelner joins from the Daily Mail where she was most recently an athletics correspondent and sports writer.
Since beginning her career at the Mail in 2011, Kelner has specialised in investigations and sports news, playing an integral role in the title's coverage of the Rio 2016 Olympics and Paralympics and London 2012.
In 2013, Kelner was the first journalist to uncover the Russian doping scandal, triggering an investigation in the UK and Russia which revealed state-controlled doping and the resultant cover up. She was highly commended at this year's Press Awards for a series of exclusives about allegations of discrimination in cycling, which rocked the sporting world.
Kelner is a former winner of the SJA's Young Sport Journalist award and has been nominated for Young Journalist at the Press Awards, as well as numerous SJA awards.
Owen Gibson, head of sport at the Guardian, said: "Martha is a talented journalist who has already broken some of the biggest stories in sport over recent years. I'm thrilled that she will be bringing her tenacity and expertise to the Guardian's award winning sports desk."
Kelner added: "I'm absolutely delighted to be joining the Guardian and am relishing the prospect of working with an editor in Katharine Viner and head of sport in Owen Gibson who I admire enormously. The Guardian is clearly one of the best sport sections around with an incredible team of reporters and writers and I'm very excited about being a part of that."
by Lydia O'Neill