The account is thought to be worth £2.5m. Mediaedge:cia and OMD also pitched for the business. The incumbent was Radar Media, which did not repitch for the account.
The win will enable Macmillan to use PHD Confidential, the agency's direct marketing arm.
PHD will be looking to maximise the revenue for the charity.
Clara Avery, direct marketing manager at Macmillan, said: "Macmillan Cancer Relief provides the expert care and emotional support that makes a real difference to people living with cancer. We offer a range of innovative cancer services and are at the heart of improving cancer care throughout the UK."
PHD will take over the account on August 16.
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