Tony Blair's former adviser, now a junior education minister, has been given the maths brief as the Government tries to head off criticism that it is not taking the difficulties facing the subject seriously.
The move comes after an influential report suggested that maths is facing a spiral of decline and that ministers have failed to act on recommendations to revive it from their own inquiries.
The report, summarising the discussions of leading academics and officials including Professor Celia Hoyles, the Government's maths tsar, at a meeting in March, paints a bleak picture.
It said Britain is no longer producing the mathematicians it needs to become future teachers, information technology workers and scientists, partly because bright pupils are not stretched enough at school.
Over the past 15 years, pupils had been steered away from more complicated, "multi-step" problems, and have instead been led through answers.
Examiners had also been under pressure from the Qualifications and Curriculum Authority not to ask hard questions in GCSEs and A-levels, said the report.
Many of these points were made in a Government report last year by Professor Adrian Smith, principal of Queen Mary college, London, which highlighted the "crisis" in maths. But this week's report said some of Professor Smith's recommendations had been watered down. He proposed a national maths centre and regional centres, at a cost of pound;30 million a year, but this has been slashed to around pound;5m a year.
No minister has had specific responsibility for maths since Ruth Kelly replaced Charles Clarke as Education Secretary in December. The maths brief for Lord Adonis, who is seen as at least as influential over education policy as Ms Kelly, will placate some who worry about the Government's commitment on the issue.
A Department for Education and Skills spokeswoman said that standards in maths were rising, that the subject was the most popular one at GCSE and third most popular at A-level, and that a new emphasis was being placed on functional maths skills at GCSE.