Browning joins the trust from the Soil Association, where she was most recently policy adviser.
She will start the new role at The National Trust in March, replacing Tony Burton who left in 2009 to head up the Civic Societies Initiative.
Browning will take on The National Trust's external affairs remit, helping ensure it is actively contributing to debates on issues ranging from heritage protection to reform of the government agricultural policy.
Director general of The National Trust Fiona Reynolds said: ‘Helen brings with her an enormous wealth of experience and has shown tremendous strength and leadership throughout her career. The trust is about bringing people and places together and Helen will play a vital role as we look to work ever more closely with our local communities.'
Browning is also an organic farmer on 1,350 tenanted acres in Wiltshire.
She said: ‘The opportunity to work with the trust is very exciting for me, partly because I will have a great deal to learn about heritage issues, but mainly because the trust is so involved in helping people enjoy and appreciate our land, history and food.
‘This has been my life's passion and focus, and this role provides a way to help the trust have even greater impact in this vital endeavour.'
The National Trust is Europe's largest conservation charity, caring for more than 300 historic houses and gardens, 1,100 kilometres of coastline and 250,000 hectares of countryside.