Cox, 41, was a staunch supporter of Britain voting to stay in the European Union, a volatile issue that will go to a referendum next week. In a June 10 tweet, Cox advocated her position.
"Immigration is a legitimate concern, but it's not a good reason to leave the EU," she wrote.
Authorities have not said what the motive for her killing was.
Hours before the attack, her husband tweeted a photo of the family in a boat campaigning to keep Britain in the European Union.
"Kids seriously disappointed there isn't another #Floatila today," Brendan Cox wrote.
But by Thursday afternoon, he went from a joking husband to a grieving widower.
"Today is the beginning of a new chapter in our lives. More difficult, more painful, less joyful, less full of love," he said in a statement.
"Jo believed in a better world, and she fought for it every day of her life with an energy and a zest for life that would exhaust most people."
He also made a promise in his wife's honor:
"I and Jo's friends and family are going to work every moment of our lives to love and nurture our kids and to fight against the hate that killed Jo."
Admired across party lines
Despite her stances that some opposed, Cox was highly respected across the political spectrum in Parliament.
Members of the opposing Conservative Party were among the first to speak about their grief and admiration of Cox.
"This is absolutely tragic and dreadful news," British Prime Minister David Cameron said.
"She had a huge heart. She was a very compassionate, caring MP. She was a bright star -- no doubt about it. A star for her constituents, a star in Parliament, and a star right across the House."
Conservative Kirklees Councillor David Hall
knew Cox for decades. The two became friends in school.
"She was a very vivacious girl, very sporty, very good at her studies educationally, very popular," Hall said.
The two later entered public office, though on opposite ends of the political divide.
Despite that, Hall said, Cox "stood for fairness." He said he saw potential for Cox to hold higher office.
"It saddens me greatly that she won't realize those ambitions, if she had those ambitions," he said.
Liberal Democrat politician Nick Clegg said Cox had a rare ability to connect with everyone, including political rivals.
"Jo Cox was unusually free of the tribal pettiness of politics -- always friendly, cheerful and kind to friend and foe alike," Clegg tweeted.
As for members of her Labour Party, Cox's death marks an enormous loss.
"It's just unbelievable," Labour Party lawmaker Rushanara Ali said. "I don't think anyone could think this could happen to such a courageous, kind and committed member of Parliament who was killed in the line of duty."
Advocate for many causes
Before Cox was elected to Parliament last year, representing Batley and Spen, the mother of two spent a decade with the aid agency Oxfam
, her website
She also worked with the Bill and Melinda Gates Foundation
and the Freedom Fund
, a charity trying to end the scourge of modern slavery.
Much of her work focused on women's and children's issues. Cox spent four years as national chairwoman of the Labour Women's Network
, encouraging more women to enter public life.
Politically, she worked for Britain in Europe, a pro-European campaign organization.
Labour politician Tristram Hunt said Cox's contributions went far beyond Parliament.
"Jo Cox was a dedicated internationalist, passionate social justice campaigner, brilliant MP, & loving mother," Hunt tweeted. "Her death is too wretched. RIP."
Her first speech to Parliament last year -- known as a maiden speech -- reflected the love and respect she had for her diverse region, made up of "independent, no-nonsense and proud Yorkshire towns and villages."
"Our communities have been deeply enhanced by immigration, be it of Irish Catholics across the constituency or of Muslims from Gujarat in India or from Pakistan, principally from Kashmir.
"While we celebrate our diversity, what surprises me time and time again as I travel around the constituency is that we are far more united and have far more in common with each other than things that divide us."
Cameron lauded her, saying she had a "great track record of caring about refugees and had taken a big interest in how we can look after Syrian refugees and do the right thing in our world. "
Cox and Conservative MP Andrew Mitchell wrote a column last year in the Guardian called "British forces could help achieve an ethical solution in Syria." She endorsed approaches on the humanitarian, military and diplomatic fronts.
"As two people who have both worked for many years at different ends of the humanitarian aid spectrum -- as an aid worker and as secretary of state for international development -- we agree on one thing: There is nothing ethical about standing to one side when civilians are being murdered and maimed. There was no excuse in Bosnia, nor Rwanda and there isn't now."
Labour Party politics
Cox dived into Labour Party politics and advocated strong leadership for the movement. She co-wrote a column with Labour Parliament member Neil Coyle in the Guardian about how she regretted nominating Jeremy Corbyn to lead the party.
"We helped put Corbyn on the ballot because we wanted a genuine debate within the Labour Party. We didn't expect to be debating things far from the priorities of most voters: unilateral nuclear disarmament, the Falkland Islands, the monarchy and all the rest. Important issues, perhaps, but not ones that swing elections. Why should we be surprised if people are turning their backs on a party that appears to have stopped talking about the things that are relevant to them?
"Weak leadership, poor judgment and a mistaken sense of priorities have created distraction after distraction and stopped us getting our message across."
She was a frank opponent of the Conservative agenda and its champions.
"Today David Cameron, George Osborne and Boris Johnson are even more determined to allow inequality, unfairness and injustice to prevail while our public services go to the wall. Every time we flounder we just embolden them further," she wrote, referring to top Conservative politicians.
First graduate, mother and wife
Cox was the first person from her family to graduate from university, her biography says.
When she wasn't in Parliament, Cox split her time between two homes: one in Batley and Spen, and one a boat on the River Thames.
Brendan Cox said his slain wife would have wanted two things above all right now:
"One, that our precious children are bathed in love; and two, that we all unite to fight against the hatred that killed her."