Kiernan joins on 4 September and will help communicate policies in the run-up to next year's national assembly election.
She takes over from Cathy Owens, who is now working for Amnesty International as programme director for Wales (PRWeek, 21 July).
The former BBC presenter and ITV Wales political editor will have a politicised role, reporting directly to Morgan.
But she is covered by the code of conduct for special advisers, and cannot attack opposition policies.
‘Jo will work alongside the central comms team but her role will be quite different from ours,' said assembly head of news Moira Trezise.