Sentencing Thomas Mair, 53, to a rare "whole life term" punishment, judge Alan Wilkie told him: "Because she was a member of parliament your crime has an additional dimension that calls for particular punishment.
"There is no doubt it was done to further a political motive. Your inspiration is not love of country, it is an admiration for Nazis and similar anti-democratic white supremacist creeds."
The jury at London's central criminal court took just 90 minutes to convict Mair of killing the mother-of-two, who campaigned for Britain to stay in the European Union, as she arrived at a library to meet constituents on June 16.
Mair showed no emotion as the verdicts and sentence were read out.
The court heard earlier that Mair -- who refused to give evidence in his own defence -- shouted "Britain first" as he fired three shots at the lawmaker and stabbed her 15 times in Birstall, northern England.
Asked to give his name at an earlier hearing, he said: "Death to traitors, freedom for Britain."
Following the verdict, Cox's husband Brendan called the murder "a political act and an act of terrorism".
"We hope the country will also take something from this," he told reporters outside court.
"That those in politics, the media or in our own communities who seek to divide us will face an unassailable wall of British tolerance," he said.
'Betrayal' of democracy
Sue Hemming, head of Counter Terrorism at the Crown Prosecution Service, said Mair's actions were "nothing less than acts of terrorism designed to advance his twisted ideology."
Jeremy Corbyn, leader of Cox's Labour Party, called the murder "an attack on democracy" that "robbed the world of an ambassador of kindness and compassion".
Investigators found an extensive collection of books on German military history, the Holocaust and Nazi race theory and a statue of the eagle of Germany's Third Reich when they searched Mair's Birstall home.
Mair had also accessed the Wikipedia page of "far right" online publication Occidental Observer and the Twitter and Wikipedia pages for Cox.
The court heard that Mair had asked the question "Is a .22 round deadly enough to kill with one shot to a human head?" during one internet search, which refers to the size of a bullet.
RELATED READING
British MPs remember Cox as Brexit controversy swirls
The killing of Cox, who had defended immigration and refugee rights, shocked Britain and led to a three-day suspension in campaigning ahead of Britain's June 23 vote to quit the EU.
Judge Wilkie said Mair had betrayed "the quintessence of our country -- parliamentary democracy.
"In the true meaning of the word she was a patriot," he added. "You affect to be a patriot."
Cox's sister Kim admitted that the 10-day trial had been "particularly challenging" and that the family was "relieved that this process is over.
"It has always been my belief that whilst we cannot always choose what happens to us, we can try to choose how we can respond," she said outside court.
"I, for one, will not be beaten by what has happened and I know I am not alone."
Mair denied Cox's murder, possession of a firearm with intent to commit an indictable offence and possession of an offensive weapon, but was found guilty on all counts.
The judged refused Mair's request to address the court personally as Cox's family watched on.
A 77-year-old local man, Bernard Carter-Kenny, was stabbed as he attempted to stop the attack.
"What you did to me that Thursday afternoon not only threatened my life but changed me as a person and all those around me," said a statement read out on behalf of the pensioner.