The Yorkshire-raised MP only entered the Commons four years ago and is 20 years younger than Owen Paterson who she replaces as Environment Secretary.
She said: “I am delighted to be appointed as Environment Secretary.
“I look forward to tackling the important issues facing our rural communities including championing British food, protecting people from flooding and improving the environment.
“I have greatly enjoyed my role at the Department for Education and would like to thank the excellent team there for all their hard work. I look forward to working with the team at Defra.”
With her brief covering food and rural affairs, Ms Truss, who was state-educated at the comprehensive Roundhay School, in North Leeds, will need to move quickly to show she has an understanding of the issues facing the countryside.
Her predecessor had won support among farmers for his full-throated defence of badger culls to try and prevent the spread of bovine tuberculosis and backing for UK food producers.
But she received early backing from the National Farmers Union through Pamela Forbes, its regional director for East Anglia where Ms Truss is an MP.
“She recognises that agriculture is a vital industry and she has taken a keen interest in food and farming issues in her constituency, particularly around food and water security.
“After the last drought she arranged for farmers to meet the Minister to discuss concerns about water supplies for irrigated crops in Norfolk and Suffolk and she has called for greater protection for farmland from flooding.”
The Country Land and Business Association paid tribute to Mr Paterson’s “great integrity”.
President Henry Robinson said: “The whole of rural England and Wales will be looking to Liz Truss to set an agenda which will assure the future of a living and working countryside. It is vital that the countryside and the businesses based there are not disadvantaged by legislation which is predominantly urban focused. We hope she wastes no time in grasping the nettle on the key issues for food and farming and presents a bold vision for our industry.”
Brought up by left-leaning parents who took her on CND marches as a child, Ms Truss went from school in Leeds to Oxford University where she was president of the Liberal Democrats before moving to the Conservatives.
A qualified management accountant, she worked at Shell and Cable & Wireless and served as a Tory councillor in Greenwich, south London, before entering Parliament.
In 2008, she was appointed deputy director of the Reform think-tank, which promoted the introduction of private sector expertise into the delivery of public services.
After two unsuccessful tilts at Labour safe seats in 2001 and 2005, she was placed on Mr Cameron’s “A-list” of talented young candidates and elected Tory MP for South-West Norfolk in the 2010 general election by a comfortable majority of more than 13,000 votes.
Her candidacy narrowly survived an attempt by traditionalist members of the constituency Conservative Association – nicknamed the “Turnip Taliban” – to deselect her after it emerged she had had an affair with married Tory MP Mark Field.
In 2012, she was appointed Minister for Education and Childcare in Michael Gove’s Department for Education. Her plans to allow nurseries to have more children per member of staff brought her into conflict with the Deputy Prime Minister who blocked the proposals.