Her responsibilities will include dealing with immigration and planning policy.
She was recently sworn in to Legislative Council for a term of 15 months alongside author Paul Craine, who both filled the two vacant seats left by Kate Lord-Brennan and Jane Poole-Wilson who were elected to the House of Keys.
They can also often be members of government departments.
The private secretary to the former Lieutenant Governor was pleased to have been appointed.
She said: ’As a newly elected MLC, it is a privilege to have been asked to support the Minister for the Cabinet Office for these detailed areas.
’I have specific interest in planning policy and past experience of immigration matters.
’I look forward to working closely with the lead officers, Cabinet Office and Tynwald members in delivering good outcomes for our island and its residents.’
Cabinet Office Minister Kate Lord-Brennan congratulated Mrs Kelsey for taking on the role.
She said: ’I am pleased that Diane has taken up the invitation to support the work of the Cabinet Office.
’Her experience, skills, interest and oversight in these particular areas will be put to good use.
Ms Lord-Brennan added: ’I am confident that she will make a positive and constructive contribution.’