The council will not reveal how much the contract is worth, but in March, when Capita was selected as preferred bidder, it was worth £290m.
The contract was awarded at the end of a three-year tendering process and councillors finally agreed on the deal at a meeting earlier this month. The plan caused controversy in the past after the Labour group accused the company of bumping up prices by insisting other services - in addition to IT - had to be included in the package. They would have cancelled the deal if they had won the next local elections.
The council said the venture will bring £25m to the city in the first three years of the partnership, which is set to transform the way residents receive services from the council.
Gateway, the current one stop shop, will be refurbished and over the next three years further service enhancements will take place, including the development of a regional business centre and call centre.
Councillor Jeremy Moulton, Cabinet Member for Resources, said: "By working with Capita the city council will benefit from millions of pounds of investment into customer services. We will also save the taxpayer millions of pounds over the term of the contract money that can be put back into frontline services."
The council will formally sign the contract early in August. Work will then start to prepare 650 staff to transfer to Capita on October 1.
Paul Pindar, Chief Executive of Capita, said: "We look forward to working in partnership with Southampton Council to transform public services in a way that will truly benefit the local community. Our extensive experience of supporting local authorities will allow us to help the Council to make step changes in service efficiency and quality."Comment on this article: firstname.lastname@example.org