The agency has been brought in to help build on the success that the Trust has recently experienced, reporting a 20% jump in visitors so far this year compared to last year.
The charity already has commercial relationships with Cadbury, BSkyB and Yorkshire Bank.
Cadbury began working with the Trust in 2008, launching the Cadbury Easter egg trails. The relationship has been extended for an additional two years and it has seen the total number of visitors to the trails this year increase 61% compared to the previous year.
The National Trust has 300 properties and more than 3.5 million members. Some 13 million people visit its venues each year.
With the recession leading more people to spend their holidays and leisure time in the UK, the Trust anticipates the number of visitors to remain at the current peak so far reached this year.
Initials was appointed by the National Trust without a pitch. It is a retained agency of Cadbury.