The Daily Mail political correspondent is quitting journalism after 12 years covering Westminster for the Mail and the Daily Express.
As associate director, Walker will be responsible for generating new business as well as advising the firm's existing clients on media handling, crisis management and campaigning.
Walker said: 'I have hugely enjoyed the past 17 years working for national newspapers and will have fond memories - particularly of working in Westminster. But I am ready for a new challenge.'
Jo Tanner, director at iNHouse Communications, said that Walker was hired for her 'political and campaigning insights'. 'Kirsty is one of the Fleet Street's finest and its loss is our gain,' she added.
Joining the Express aged 21, in 1998 Walker was appointed Fleet Street's first drugs correspondent before becoming the paper's social affairs correspondent. She joined the Mail in 2006.
The agency has also hired Stephanie Reeves, formerly of Insight Public Affairs, as a senior consultant.