Rt Revd and Rt Hon Lord Williams of Oystermouth takes over from Desmond Tutu, the Bishop of Southern Africa, who announced he'd be stepping down last year. Lord Williams has been the Vice-President of the Church Army since 2005.
The Church Army is a Christian evangelism organisation that operates in the UK and Ireland. It's started 132 years ago, in 1882.
One of its ventures includes the Marylebone Project, which is one of the biggest providers of housing and services for homeless women in the UK.
Lord Williams said: "I am deeply honoured to be asked to take up this position.
"There can be few organisations that have done as much as Church Army to support the highest quality of innovative ministry and outreach in our Church and to encourage lay and clergy alike in their calling to speak and act the Good News effectively and imaginatively.
"I look forward with great enthusiasm to a closer connection with Church Army and to supporting all its members as best I can in prayer and solidarity."
Church Army Chief Executive, Mark Russell, said: "I am delighted Lord Williams has agreed to serve as Church Army's new President.
"Rowan Williams has been a good friend to Church Army and to me personally for many years.
"He is passionately committed to what we do, and we are delighted he has this new link with Church Army."