The charity has brought in the agency to help raise awareness of the importance of such donations in funding its work as it launches a discounted will writing service.
While more than 70 per cent of people in the UK give to charity, less than a tenth of these then leave money to charity in their wills, according to Macmillan Cancer Support.
Helen Eddleston, legacy manager at the charity, said: "The gifts Macmillan Cancer Support receives from people in their wills are both vital and invaluable in helping us to ensure no one has to face cancer alone.
"They make up a third of all of our income – no matter what size, every legacy helps Macmillan provide vital support to people affected by cancer.
"We have decided to work with Calacus because we are confident it will help us to create talkability and quality conversations around legacies and will-writing, which can often be seen as a ‘taboo’ subject."
Calacus MD David Alexander said: "There is a saying that ‘there is nothing certain in life apart from death and taxes’ and while encouraging people to think about their own mortality is something of a taboo, legacy gifting is such an important source of revenue for charities."
Calacus won the brief after a competitive pitch involving two other agencies.