David Miliband, the former foreign secretary, will resign as an MP to become head of the US-based refugee charity the International Rescue Committee.
The Labour MP for South Shields, whose brother Ed beat him to the party's leadership in 2010, will take up the joint roles of president and chief executive on 1 September.
The charity is based in New York but has branches around the world, including the UK. It employs 12,000 people worldwide and is expected to have an income of about £300m in its current financial year, which will end in October.
A spokeswoman for IRC-UK said details of Miliband’s salary were confidential. The charity’s accounts say that George Rupp, his predecessor, was paid about £260,000 in 2010/11.
Miliband, who worked at the National Council for Voluntary Organisations before going into full-time politics, said in a statement that he would bring to the IRC a "passionate commitment to social justice and international cooperation".
"The IRC’s mission is personal for me because my own parents were refugees who arrived in Britain in the 1940s," he said. "I look forward to honouring the memory of those who helped my parents by leading the IRC as it continues to offer help to uprooted people around the world."
The former US president Bill Clinton said in a statement that Miliband was one of the "ablest, most creative public servants of our time".