The annual Christian conference has been going since 1875, putting on Christian seminars and Bible teaching each summer for thousands of people.
The Lake District National Park Development committee unanimously voted to support the planning application for the renovation.
The entire convention will move to the site of the former Pencil factory, with old warehousing being cleared to provide space for the convention's main marque.
They currently meet at a show ground but also in churches and meeting spaces across the town.
The Pencil Factory building will be modernised to provide toilet facilities, accommodation, meeting rooms and a conference centre.
The refurbishment will also host Keswick Ministries' teaching courses and be open for visiting church groups, schools and local community groups.
The space may also be used by other organisations from around the country.
James Robson, ministry director, of Keswick Ministries said: "This is a wonderful milestone in our charity's 140-year history and for the town of Keswick.
"The site, which has been derelict for ten years, will be transformed into a facility that the town and the people of the Lake District can be proud of.
"Our vision is to create an even better Convention experience for supporters on one site with improved facilities, plus a modern venue in this World Heritage Centre that will attract many other visiting groups throughout the year. This project will allow us to strengthen the work we are doing as well as provide a fantastic local facility."
Keswick Ministries say the Convention brings considerable economic benefits to the town and the local area.
It is estimated that each summer the thousands of people who attend the convention bring over £3m in spending to the area.
Work will start as soon as final planning permission is granted and it is planned that the first convention will be held on the site in 2020.
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