West Dunbartonshire Council has finalised contracts for the project following funding wrangles.
The school was one of 12 government projects across Scotland delayed while civil servants sought technical advice over the funding arrangements.
The issue was resolved late last year and council officers have been working hard behind the scenes to get the project ready for financial close which is now expected by March 31.
The school in Bellsmyre is likely to start on site in April and could be open to pupils as soon as October 2017.
The 1000-pupil school has already been granted planning permission so work can begin as soon as contracts are concluded.
Councillor Michelle McGinty, Convener of Educational Services, said: “This is great news. Pupils, parents and staff have waited a long time for this new school and I’m delighted that we are now set to go on site and finally begin building a new our Lady and St Patrick’s High School.
“Building a new OLSP has remained a top priority for this Council and will mark the completion of the regeneration of the secondary school estate in West Dunbartonshire which has moved thousands of young people from sub-standard buildings into modern, purpose built learning environments.”
The development is being fully funded by the Scottish Government.
The Council has set aside £550,000 over the next two years to cover costs which are not associated with the construction including the demolition of the existing school in Castlehill.