Boris Johnson has described US President Joe Biden as a "breath of fresh air" after they met for the first time on the eve of the G7 summit in Cornwall.
"There's so much that they want to do together with us, from security, NATO, to climate change," Mr Johnson said.
He also said there was "complete harmony" with Mr Biden over Northern Ireland.
The US, UK and EU all want to protect the Good Friday Agreement, the PM said.
It comes after Mr Biden had earlier warned that the UK-EU dispute over border controls should not risk the peace process in Northern Ireland.
The Good Friday Agreement, a peace deal signed in 1998, helped end decades of violence in Northern Ireland.
Mr Johnson said he was "optimistic" the peace process would be kept going and that there is "absolutely common ground" on the issue between the two leaders.
During their meeting, the two men also established a taskforce to re-establish travel across the Atlantic, after the US banned most British people from entering at the start of the pandemic.
And they agreed a deal - labelled the "Atlantic Charter" - which commits the two countries to work together on global challenges.
Mr Johnson said: "It's incredibly important that we should affirm that [relationship] and the talks were great, they went on for a long time, we covered a huge range of subjects, and it's wonderful to listen to the Biden administration and to Joe Biden."
Mr Biden said the "Atlantic Charter" would address the "key challenges of this century - cyber security, emerging technologies, global health and climate change".
"We affirmed the special relationship - that is not said lightly - the special relationship between our people and renewed our commitment to defending the enduring democratic values that both our nations share," he added.
Earlier, the leaders posed for photographs with their wives and the four exchanged gifts, with keen cyclist Mr Johnson receiving a US-made bike and helmet.
Mr Johnson gave the president a photo of Frederick Douglass - a former slave who campaigned against slavery in the 19th Century - and a first edition of Daphne du Maurier's The Apple Tree, a collection of short stories, to First Lady Jill Biden.
The G7 summit begins on Friday and will be the first time world leaders have assembled in person since the coronavirus pandemic.
They will be joined by the Queen, the Prince of Wales, the Duchess of Cornwall, and the Duke and Duchess of Cambridge, who will all attend the G7 leaders' reception on Friday evening.
Afterwards, Prince Charles will host a reception for the G7 leaders and CEOs from the world's largest companies to discuss how the private sector can work with governments to tackle the climate emergency, with Prince William also attending.
Covid vaccines and climate change will be among the main issues on the G7 agenda.
But the on-going disagreement between the UK and the EU over post-Brexit regulatory checks on goods going into Northern Ireland from Great Britain also look set to feature heavily in diplomatic discussions.
The arrangements were agreed in the 2019 Brexit withdrawal deal, but the UK has since sought more flexibility.
Asked about the dispute on Thursday evening, French President Emmanuel Macron said: "I think it's not serious to want to review in July what we finalised after years of debate and work in December.
"We have a trade deal - it has been painfully discussed for years... if six months later, they say: 'What we negotiated with you, we don't know how to respect it', then that means that nothing is respectable anymore."
European Commission president Ursula von der Leyen said the Northern Ireland protocol - the name for the post-Brexit trading rules - was the "only solution" and should be implemented fully.
What is the UK-EU Northern Ireland dispute about?
Northern Ireland was given special status as a result of the 2019 Brexit "divorce" settlement between the UK and the EU.
While England, Scotland and Wales no longer follow EU rules, Northern Ireland still does, because it shares a land border with the Republic of Ireland, an EU member.
In order to avoid a physical border between the two countries - and thereby protect the peace process - it was agreed that customs checks would take place on goods entering Northern Ireland from elsewhere in the UK.
But unionists say this has effectively put a border down the Irish Sea instead - something they are ideologically opposed to - and business say supply chains have been complicated and disrupted.
Talks on Wednesday between Brexit minister Lord Frost and the European Commission's Maros Sefcovic to try to resolve the standoff ended without a breakthrough.
Since becoming president, Mr Biden has frequently stressed the importance of the Good Friday Agreement, which was brokered by the US.
And speaking before the summit, the president's national security adviser told the BBC that Mr Biden has "deep" concerns that a UK-EU trade row could endanger peace in the region.
Asked if Mr Biden made his alarm about the situation in Northern Ireland clear, Mr Johnson replied: "No, he didn't."
Before the meeting, Mrs Biden and Mrs Johnson joined their husbands in Cornwall for a walk in Carbis Bay.
Admiring the view, Mr Biden said: "It's gorgeous - I don't want to go home."
The two women dipped their feet in the sea and later had tea together.
Dr Biden told reporters: "It's really nice to be here in Cornwall. It's my first time. Obviously it's beautiful for those of you who have been here before."
Mr Biden suggested that both he and the prime minister had "married way above our stations" - something with which Mr Johnson agreed.
The prime minister married his partner Carrie in a low-key wedding in Westminster Cathedral, at the end of May.