Councillors voted unanimously to declare a climate emergency in St Albans in a meeting last night.
A petition from Sustainable St Albans, which called on the district council to make the declaration, reached 1,700 signatures, warranting a debate by the full council.
Campaigners protested outside the Civic Centre ahead of the meeting, urging councillors to take action against the climate crisis.
Sustainable St Albans trustee Catherine Ross spoke at the meeting alongside 17-year-old Mimi from St Albans Extinction Rebellion.
Catherine, who was also chair of the 2019 Sustainability Festival, said: "It's quite literally, although gradually, a matter of life or death that you consider tonight.
"Tonight isn't about any one action, it's about agreeing a direction of travel. There's great symbolism if every one of you votes across party lines to declare a climate emergency.
"You can't solve a crisis without calling it a crisis."
They also called for the council to pledge to do everything in its power to make the whole district carbon neutral by 2030.
The petition was jointly organised by Sustainable St Albans and St Albans Friends of the Earth, who collected signatures during the Sustainability Festival.
Members of both groups joined St Albans Extinction Rebellion, St Albans Labour Party and St Albans Green Party in the protests outside the council offices.
In the meeting, council leader Cllr Chris White said: "Thank you to the petitioners and indeed to the tuneful protesters that we encountered outside the building.
"This is deadly serious. This is an emergency; it's not just a problem. This has come up as a more pressing issue than Brexit and one that's more capable of uniting the council."
Labour Cllr Roma Mills also suggested action the council could take beyond the district itself, such as looking at the environmental impact of flights from nearby airports and sharing ideas for protecting the environment with twin towns in other countries.
All 55 councillors at the meeting, from across political parties, voted in favour of declaring a climate emergency.
Catherine said: "It is a brilliant result - and so positive that there was cross-party agreement in favour of declaring the climate emergency in our district.
"I just want to thank all of the residents, councillors and officers who helped to make this happen - and to say that Sustainable St Albans will help in any way we can as the council goes forward in developing an action plan.
"Now the work begins."
Across the UK, 120 councils have declared a climate emergency, and 70 of these, like St Albans, have set a target date of 2030 to go carbon neutral - 20 years sooner than the national target of 2050.
Cllr White told the Herts Ad: "It's [the Liberal Democrats'] number one priority and it's now the council's number one priority.
"I think what's remarkable is how public opinion has changed. It's become a general worry amongst the population.
"All councils need to react - we are not pioneering in this or doing this to become pioneering.
"I am delighted that the parties have come on board on this with enthusiasm."
The council also agreed a motion put forward by Cllr Mary Maynard to ban the use of single-use plastic in council offices, and work towards banning its use in facilities licensed by the council and at events supported by the council or held on council property.
This follows the setting up of a new Plastic Free Harpenden group, which joins Plastic Free St Albans in calling for a reduction of the use of single-use plastic across the district.
Christelle Garcia, from Plastic Free St Albans, said: "It's fantastic that the council is supporting the reduction of single use plastics in such a practical way that should see quite an impact in the district.
"Local businesses have already been very pro-active on this issue and this motion will provide a huge boost to the movement to remove these unnecessary plastics from our everyday lives."
Sustainable St Albans trustee Gail Jackson said: "This was a remarkable and historic night for the district with two major motions supporting environmental sustainability.
"Over 5,000 people took part in the events during the fourth Sustainabilty Festival and these concerned residents must now feel that their voices have been heard."
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