The charity wants its communications to present a consistent message, whether they are addressing opinion formers or potential donors in the run-up to its 60th birthday in 2006.
The appointment, made without a pitch, follows the arrival of Deborah Hamilton-Lewin as Mencap's marketing director at the end of last year.
Hamilton-Lewin was previously at Heart of the City, an initiative backed by the Bank of England to encourage City companies to increase their commitment to corporate social responsibility.
Influence, which launched at the beginning of the year, models itself on US companies that provide strategic advice on complete communications programmes.
Founded in 1946, Mencap is the largest UK charity in its field and provides a range of direct support services for people with learning disabilities, their families and carers.
The charity lobbies extensively as part of its efforts to win equal rights for those with learning disabilities. Mencap uses direct marketing to help raise funds, but has not advertised above the line for some time.
Mencap recently teamed up with the internet service provider AOL UK in an attempt to increase awareness of its fundraising initiatives.
However, Mencap faces the perpetual challenge of making its voice heard in an increasingly crowded charity sector.
Damian Collins, the director of Influence, said: "We're working on a strategy that will work across all communication channels. This could eventually evolve into above-the-line advertising, but that's not our starting point."