Deputy Prime Minister Nick Clegg described Mr Johnson's move into a ministerial role as "a political appointment for the Conservative Party".
Mr Clegg told LBC 97.3 radio:
This is a political appointment for the Conservative Party, if you like, about how the Conservative Party generates policy ideas, and I don't think you can say the Prime Minister as leader of the Conservative Party is not entitled to appoint a Conservative MP to come up with policy ideas for his party.
Stanley Johnson, the father of London Mayor Boris, told ITV News he was "absolutely thrilled" that his son Jo has been added to David Cameron's team in Downing Street.
He said: "I'm absolutely thrilled at this wonderful appointment David Cameron has made, there is no doubting Jo's intelligence.
"He is delighted and I'm confident the Tories will get their act together in a big way.
"I wish my father had been alive to see this and I'll keep lobbying him to make sure the Tories stay green."
David Cameron has tweeted a picture of his new policy board, which features London Mayor Boris Johnson's younger brother Jo.
The Prime Minister's tweet said: "I've been meeting with my new policy board, made up of MPs. I told them I want more ideas to help hardworking people."
The Daily Telegraph's political correspondent Peter Dominiczak has tweeted London Mayor Boris Johnson's reaction to his brother Jo heading up a Policy Unit in Downing Street.
Boris on his brother's promotion : "Great there is finally a Johnson in Number 10. Jo will be superb. Now for Rachel and Leo!"