Mrs Brown worked closely with the 41-year-old when the future Batley & Spen MP was a leading campaigner with international aid charity Oxfam.
“I am heartbroken. Jo had a truly remarkable spirit and passion that shone through in her work with Oxfam and with me on our countless campaigns for women and children,” said Mrs Brown in a poignant tribute.
“Jo cared about everybody but she reserved a special place in her heart for the most vulnerable and the poorest citizens of the world.
“She was fearless, she was endlessly upbeat and she reached out to so many to join her cause. Her mission was to make the world a better place.
“But above all else Jo had utter devotion to her husband Brendan and their two children Cuillin and Lejla.”
Jo Cox worked for Oxfam and Oxfam International between 2001 and 2009 in a variety of different roles and met her future husband during her time there.
As head of Oxfam’s Brussels office she spearheaded the charity’s campaign for trade reform. In 2005 she joined Oxfam GB as head of advocacy.
The charity said she was a passionate advocate on humanitarian issues including the conflicts in Darfur and the Democratic Republic of Congo.
It said her commitment to humanitarian issues led her to become head of humanitarian campaigns for Oxfam International in New York in 2007 for two years.
Max Lawson of Oxfam, who worked closely with her, said: “Jo was a diminutive pocket rocket from the North. She was as a ball of energy, always smiling, full of new ideas, of idealism, of passion. She gave so much to Oxfam. She was particularly brilliant at bringing huge energy to our campaigning around the desperate humanitarian crisis in Darfur.”
Oxfam GB’s chief executive, Mark Goldring, said: “Oxfam is proud of the role that Jo played in our work over a decade. Many of our colleagues remember her fondly. The rest of us followed her work with admiration. She never lost her passion for peace, justice and equality. Everyone is deeply shocked to hear the news.”