Mr Stoltenberg said the meeting comes "at a pivotal moment for our alliance, and today we'll open a new chapter in our trans-Atlantic relationship".
It is the first NATO summit involving US President Joe Biden.
The behaviour of Russia will feature heavily in talks between leaders at the summit, ahead of Joe Biden's meeting with the country's president Vladimir Putin in Geneva on Wednesday.
Many allies are hoping to secure Mr Biden's assurances that the United States will stand by them in times of conflict.
NATO faced turmoil for four years under former president Donald Trump.
President Biden told an audience of US military personnel and service families: "We are going to make it clear that the United States is back and democracies of the world are standing together to tackle the toughest challenges and the issues that matter most to our future; that we're committed to leading with strength, defending our values, and delivering for our people."
Watch: President Biden speaks to personnel at RAF Mildenhall last week.
On his NATO meeting, he said: "In Brussels, I will make it clear that the United States' commitment to our NATO Alliance and Article 5 is rock solid.
"It's a sacred obligation that we have under Article 5. "The US and the UK are both founding members of NATO – the strongest military and political alliance in the history of the world.
"Our troops have stood shoulder-to-shoulder around the world, including serving bravely in the mountains of Afghanistan for the past 20 years.
"Our NATO allies have had our backs when it mattered, just like we've had theirs when it's mattered.
"And now we need to modernise our alliance, investing on our critical infrastructure, our cyber capabilities, and to keep us secure against every threat that we've faced over the last decade, and the new challenges we're about to face as well," he added.
On his meeting with Vladimir Putin, Mr Biden said he will "let him know what I want him to know".
On 10 June, following his first meeting with President Biden, Prime Minister Boris Johnson said: "It's wonderful to listen to the Biden administration and to Joe Biden because… there's so much that they want to do together with us, from security, NATO, to climate change, and it's fantastic.
"It's a breath of fresh air."
Last May, US President Donald Trump called for an expansion of the group's membership because he considered it an outdated body that does not properly represent "what's going on in the world".
Mr Trump singled out Russia, Australia, South Korea and India as possible additions.
In April 2019, President Trump said he was committed to NATO, but also repeated his stance that the "alliance cannot rely on one nation to defend all".
"If you look at it, the disproportionality of what the United States is doing is really too great, but we're working on that," said the then-President.
In January 2017, he went so far as to brand NATO "obsolete", claiming it has not tackled terrorism, before stating that the military alliance was "very important" to him.
Cover image: Sipa US/Alamy.