The process is being run by AAR and Mencap expects to have a creative agency appointed by early June.
A request for information has been sent to agencies as the charity looks to reposition its brand. A shortlist of agencies will be invited to pitch.
Until now Mencap has produced direct response TV ads, including this week's "help a child shine" campaign, which were created in-house by the charity and DTV, the DRTV specialist agency.
The charity recently appointed John Ayling & Associates to handle its media planning and buying requirements and this appointment will be unaffected by the creative review.
Gerard Cousins, the director of marketing and engagement at Mencap, said: "Whilst there is a great awareness of the Mencap brand, there is insufficient understanding of exactly what we do and who we help.
"We are looking to appoint an agency to help us propel the Mencap brand to the front of people’s awareness so that we are the first charity that they think of when it comes to learning disability".