His letter responded to another letter from Angela Rayner, deputy leader of the Labor Party.
Mrs Rayner asked Mr Case a number of questions about his involvement in the alleged job search, including whether he had spoken to the Foundation about the role.
He also asked if Mr. Johnson had raised the issue with the Queen or any other member of the royal family.
Ms. Johnson eventually held a job as director of communications at the Aspinall Foundation, an environmental charity.
In her letter to the cabinet secretary Thursday night, Ms Rayner said: “There are now serious questions for the prime minister to answer about possible improprieties and conflicts of interest in his lobbying for a well-paid job. and funded with taxes for her partner in 2018., Carrie Johnson.
“Reports suggest that while the Secretary of Foreign Affairs, Mr Johnson tried to get Mrs Johnson appointed for a job in the Department of Foreign Affairs in 2018 and for other functions, including one with a charity real “.
Answer by Simon Case
Mr Case said: “I was informed that there was a publicly announced role and possible volunteer opportunities.
“According to the charitable status of the Earthshot Award, the publicly announced paper was not publicly funded and any applicant would have to go through a full and open interview process.
“I don’t know how this information was used, but as far as I understand, Ms. Johnson did not take advantage of any of these opportunities.”
Angela Rayner asks for a new ethics advisor
In a separate letter to Lord Evans, chairman of the Public Living Standards Committee, Ms Rayner asked officials who were allegedly involved in trying to get a job to Mrs Johnson to present public evidence.
He said Ben Gascoigne, one of Johnson’s closest aides, and Dominic Cummings, his former chief adviser, should be brought before investigators.
He also called for the appointment of a new emergency ministerial ethics adviser soon to prevent Johnson from being “a judge and juror in his own case.”
Downing Street has denied that Johnson recommended his then-girlfriend.
His spokesman has denied that he was given to the Foundation for work.