Mr Knight, the MP for Dorset South since 2001, has been appointed Jacqui Smith's successor as Labour's seventh schools minister.
The post will give the 40-year-old Cambridge graduate responsibility for a portfolio which covers secondary-school reform, behaviour, the 14-19 agenda, workforce remodelling, the secondary curriculum and raising standards.
He and his new boss, Alan Johnson, the Education Secretary, went straight into action on the education Bill this week. The two men have played together in the Lords and Commons tennis team. "I'm Tim Henman to his Andy Murray," Mr Johnson said.
Mr Knight's first government post was as a parliamentary private secretary in the Department of Health before joining Defra as minister for rural affairs, landscape and biodiversity last year.
Of his appointment, Mr Knight said in his personal website diary: "The call from the PM came in the middle of my constituency surgery. I was surprised and hugely flattered to get such a high-profile job."
Elsewhere in the Department for Education and Skills, Bill Rammell remains as higher education minister, with Phil Hope his understudy as skills minister. Lord Adonis remains as minister for schools. Beverley Hughes, the children's minister, keeps her job, although she now has a new number two, Parmjit Dhanda, the 35-year-old Gloucester MP, who has been appointed minister for young people and families, replacing Maria Eagle.
Mr Dhanda, who used to work for Connect, the union for communications professionals, was a parliamentary private secretary to Stephen Twigg in 2004.