Jens Stoltenberg was speaking as leaders met in Brussels for the NATO Summit.
The NATO chief wanted the focus to be on the future, but it was clear that the past and present would be discussed too.
UK Prime Minister Boris Johnson said: "I'm always hopeful that things will improve with Russia, but I'm afraid that so far, it's been pretty disappointing from the UK point of view."
It could explain why NATO has made Russian language videos, promoting the 'NATO 2030' plan, which was agreed on Monday.
The new mission statement outlines how NATO wants members to team up on the resilience of their critical national infrastructures, such as 5G data networks and undersea internet cables, and to see climate change as a threat with military implications.
Dr Karin von Hippel, Director General of the Royal United Services Institute (RUSI), said: "Whether it's cyber, whether it's new technologies, AI etc, countries need to come together to manage it.
"So NATO does need to evolve.
Watch: Boris Johnson says NATO is not seeking a 'new Cold War' with China.
"I think it is important for every alliance, if it's going to stay critical and important and relevant, to redefine itself. I think we're seeing with the UN that it's not doing that in the same degree, in the UN it's falling behind."
Reinvention has proven challenging amid four turbulent years for NATO
For the new US president, this summit has been about trying to reassure allies that stability is back, so they can make these sorts of long-term plans.
President Joe Biden said: "I want to make it clear – NATO is critically important for US interests in and of itself. If there weren't room, we'd have to invent one. I just want all of Europe to know that the United States is there."
The president does share some ground with his predecessor Donald Trump, with both concerned about China – a view shared by NATO itself.
China is explicitly mentioned in the alliance's new strategic concept – the first time in its 72-year history.
Mr Stoltenberg added: "NATO leaders called on China to uphold its international commitments and to act responsibly in the international system, including in space, cyber and maritime domains."
It marks another significant change in NATO's remit – the decision is not about an immediate conventional military threat to the alliance and is only partly about China's military growth.
Dr Julie Norman, Deputy Director, UCL Centre on US Politics, told Forces News: "Right now, there's a lot of concerns over what people see as changing the rules regarding international trade, not abiding by the world rules and orders in terms of intellectual property, in terms of trade rules, as well as the internal issues with the persecution of the Uighur community, with Hong Kong."
So how is it relevant to the military alliance?
Dr Norman added: "Biden is trying to posit this view of democracies versus authoritarian regimes. I think some in NATO might push back at that black-and-white thinking a little bit and see a bit more nuance there.
"For China right now, a lot of the responses and policy are not going to be military in nature, it's going to be more in terms of economic pressures, political pressures, there won't be a push to have any kind of military response to China unless something changes drastically."
Russia, however, is still NATO's biggest concern. Defending Europe is its biggest task as Moscow creates new military units on the alliance's outskirts.
NATO requires more money as it aims to increase its activity in the future. There is a new pledge for more funding – not just the 2% target for national defence budgets, but members will provide more for the alliance's core budget.
A portion of it will help to develop new military technologies.