Mr Prentis won 125,584 votes against Roger Bannister's 71,021 and Malkiat Bilku's 27,785. He is currently deputy general secretary under Rodney Bickerstaffe, and will take over on 1 January when Mr Bickerstaffe stands down.
At the press conference announcing his victory, Mr Prentis called for more government money for public services and an end to privatisation and the private finance initiative.
He said he would continue Mr Bickerstaffe's work in demanding more public money: 'We will be calling for far greater investment in public services. We'll continue our campaign against the private finance initiative. We want to sit down and talk about how we can fund public services properly. We want a forward-looking agenda we can commit ourselves to and the government can commit themselves to.'
'Like Rodney, if the government takes action which is not in the interests of our members and the services they provide I will not hesitate in criticising it.'
He said best value was the up-and-coming issue in local government, and warned of industrial relations flare-ups if it was imposed without workers being taken into consideration.
'Who can oppose best value if it actually means the workers and employers get together and work out a better way of providing services? Unison has got
to be an organisation which looks to improve services.
'[But] if the government and local councils believe they can achieve change through force or through not involving the workforce then it won't work. Local government workers have been through reorganisation, they have had CCT cutbacks. They're not afraid of change, it's just that every change that's been made has affected their pay and conditions.
'Unless the government gets pay and conditions right it could fall into the trap of imposing [best value]. In the past year only 51% of councils in best value exercises have actually consulted staff.'
He was concerned that the guidance on best value put the emphasis on externalisation and competition. He warned there was concern among activists that best value would be used as an excuse to cut pay and conditions, as some councils had already done.
He denounced PFI as a poor use of money. He cited Pimlico School in Westminster, which needed£7m for essential repairs. 'The government offered them£2.5m but said if they build a PFI school they could have£21m. That's money that isn't used somewhere else. All they wanted was£7m. Can we afford the scale of the undertakings that go along with PFI?'
Although Unison backed Ken Livingstone's bid for the Labour London mayor candidature he said Mr Livingstone should probably not run as an Independent. 'The issue that really concerns me more than anything is whether or not Ken, having accepted the rules of the game, should now call foul after the result. Perhaps it would have been more honest to have done it before the result was announced.'
He did not think mayors were necessarily a good idea, preferring a leader elected by and accountable to an assembly. But it was too early to make a staffing issue out of how mayors might work.