The Duke has regularly spoken out about climate change issues.
On Thursday he called on governments to do more to protect the environment from companies pursuing profit from fossil fuels in Africa.
The Duke and Mr Mangundu said: “To protect the Okavango River Basin, we call on the world to stand in solidarity with us, our allies and local communities in advocating a full moratorium on oil and gas development in the region.”
They added: “Now, the choice is simple: Either we honor our natural and life-sustaining ecosystems, preserving them for generations to come, or we exploit them on a path to permanent destruction.
“Will you stand with us?”
The delta is one of the largest freshwater wetlands in southern Africa, the main source of water for a million people, and one of Africa’s richest places for biodiversity.
Ahead of a visit to the region in 2019, Harry said in an Instagram post from his official account: “Known by the locals as ‘source of life’, this ecosystem is wilderness at its best, playing an absolutely crucial role for the planet, people and wildlife. This is our one and only chance to save this magnificent last Eden.”