Labour gained 11 seats taking a majority control of the council for first time in more than a decade.
As the final declaration came in just after 2am, Labour councillors and supporters gathered to chant out their new number of elected members - 30.
The result decides who will run the city council for the next two years, as there are no elections next year.
After a bitter election fight Labour councillors, backed by a union campaign, were confident of snatching power from the Tories who have run civic affairs for the past four years.
Labour leader Richard Williams, who held his Woolston seat, said: “It's a great night. I'm just relieved and pleased. We worked hard and spoke to over 30,000 people.
“A great deal were unhappy about the way the city has become a mess and seemed to be voting on a national level on issues around the economy."
Dejected Tory leader Royston Smith said: "It's a very disappointing night. But you only have to look at the results from the last few years to know the result was not especially surprising.
“We clearly laid out our programme and theirs (Labour) doesn't stack up. We couldn't have worked harder."
He said a public backlash against the strikes over pay cuts last summer was a "red herring".
Eighteen seats across all 16 wards were being contested, with double headers due to resignations in Bitterne Park, won by the Tories, and Peartree, won by Labour.
Labour, who had 19 councillors, needed to hold two seats and gain six to take overall control of the council for the first time since 1999.
They romped to victory with 11 gains, taking ten seats from the Tories and one former Lib Dem seat.
Conservatives, who swept to power for the first time in 24 years the last time the same seats were contested in 2008, held 25 of the 48 seats on the council, a majority of just one going into the count. They are now down to seat sixteen seats.
Lib Dems remain on two seats.
Conservative city mayor Terry Matthews was left red faced after losing his seat by 351 votes to Labour’s Mark Chaloner.
Labour youngster Dan Jeffery, 22, took the Tory’s seat in Sholing, where jet set former councillor Neil Fitzgerald stood down after landing a cruise job in Los Angeles in January year, but continued to claim his allowances and represent residents while living 5,000 miles away.
Among its other gains Labour also took Coxford and Redbridge, giving them a full complement of three councillors in each ward.
Millbrook was won by Georgie Laming, who became the youngest ever female city councillor aged just 19.
The Solent University student was jubilant. "We put a lot of hard work into the ward for three or four years. I'm glad it's all paid off. I'm delighted."
In Bevois, Labour’s respected former finance spokesman Stephen Barnes-Andrews retained his seat but will serve on the backbenches.
Tory young gun David Fuller, who was elected as a Itchen College student aged 18 four years ago, lost his Bitterne seat to Labour’s Mary Lloyd.
The Tory's leisure and finance boss, councillor John Hannides, held Bassett comfortably, with nearly half of the vote. Councillor Ivan White, the Tory’s adult heath boss and a party chief in Itchen, also held his seat in Bitterne Park.
Outgoing council leader Cllr Smith was spared a loss in his own backyard. Edward Daunt held off a Labour challenge to his Harefield seat by 142 votes.
The next leader of the council will be elected to the £30,000 a year post on May 16, when the new Cabinet will also be confirmed.
The task of dealing with an estimated £46m budget black hole over the next two years will then begin.
Labour have promised no more redundancies at the council and to restore staff pay cuts brought in by the Tories last summer in a bid to avoid even more lay offs.
The cuts sparked rolling strikes by bin men and street cleaners that left rubbish piling up in the city for 12 weeks last summer.
Labour, led by councillor Williams, are also promising more jobs and housing to spur growth and a new Southampton festival this September.