Work will start on site next April and is due for completion by September 2017.
The project will see a brand new school and college for 11-18 year olds built on land formerly used by Cardiff and Vale College for its Trowbridge campus.
Cllr Sarah Merry, Cabinet Member for Education, said: “This represents a £25m council investment in the future of children growing up in the east of the city, and using Willmott Dixon to design and build the new facilities will ensure the best possible education for pupils and students in East Cardiff and the best value for money for the taxpayer.”
Willmott Dixon managing director for Wales, Neal Stephens, said: “We are delighted to be involved in this excellent new school for the city and given that so many of our staff live here, it will be something their children will benefit from too.
“Community will be a big focus for us and we will try whenever possible to use local labour and companies to build Eastern High.
The new school is being jointly funded by the City of Cardiff Council and Welsh Government, as part of the 21st Century Schools programme.
Cardiff and Vale College Principal and Chief Executive Mike James said: “This development will create a fantastic facility that the whole community of East Cardiff will benefit from.
“It will provide us with the opportunity to inspire and excite young people to learn, achieve and progress in a brand new building with a wider choice of learning opportunities than ever before.”