Harry Dunn’s mother said the family would “continue our fight” after the US refused to extradite the woman who is to be charged with causing his death.
Charlotte Charles said she was “fairly calm” following the decision, adding it was “what we expected”.
Mr Dunn, 19, died after a crash on his motorbike outside RAF Croughton in Northamptonshire in August.
Mrs Charles said: “We’re not going anywhere. The extradition request will forever be over Anne Sacoolas’ head.”
“We had mentally prepared ourselves and have got plans in place to try and deal with [the decision],” she added.
Mrs Sacoolas, the wife of a US intelligence officer who worked at the base, left the country under diplomatic immunity following the crash.
Extradition proceedings were launched earlier this month.
The US Secretary of State Mike Pompeo turned down the extradition request in an email to the UK Government on Thursday evening.
Mrs Charles said: “We’re quite happy to continue our fight. Nothing is going to stop us, nothing is going to make us go away, it does not matter how long it takes.
“If it means we have to wait for the next [US] administration then we will.”
The prime minister’s spokesman said the government would “carefully consider” what future action could be taken and was “urgently considering” options.
He added the Crown Prosecution Service was considering the legal position.
Andrea Leadsom, who is the MP for South Northamptonshire, where Mr Dunn’s family live said: “This is clearly wrong and the decision must be changed.
“At its heart, this was a road traffic accident where a lovely young man, with his life ahead of him, died: diplomatic immunity should not be at play here.
“The person who has been charged by the CPS must be bought back to the UK to stand trial.
“We should stand shoulder to shoulder with Harry’s family to get justice done and I can tell the US State Department that the Dunn’s are completely heartbroken.”
Foreign Secretary Dominic Raab said he had told the US Ambassador the government was “disappointed” about the decision and that the UK “would have acted differently” had the crash occurred in the US and involved a UK diplomat.
“We feel this amounts to a denial of justice, and we believe Anne Sacoolas should return to the UK,” he said.
Mrs Charles said she was “extremely pleased” the government had “shown their full support” to the family.
Speaking to BBC Radio 4’s Today programme, Dunn family spokesman Radd Seiger said the latest move had been “factored it into our planning and strategy”.
“The reality is that this administration, which we say is behaving lawlessly and taking a wrecking ball to one of the greatest alliances in the world, they won’t be around forever whereas that extradition request will be,” he added.
“We will simply plot and plan for a reasonable administration to come in one day and to reverse this decision.”
Mr Seiger said it was “the first time in history that the United States has turned down an extradition request”.
On its website, the US Embassy says it “has not denied a single extradition request from the UK under the treaty” while the UK has denied 10 requests from the US.