The agency won the business after a competitive pitch against Zed, Big MouthMedia and Agenda 21.
Macmillan put the account up for pitch following the closure of its previous agency, i-level, in May.
BigMouthMedia, which handles the charity's search business, had handled the display activity in the interim period.
Eva Lake, the digital marketing manager at Macmillan, managed the pitch process.
The appointment of Vizeum does not change PHD's status on the charity's offline media planning and buying business.
Macmillan, which provides practical and emotional support for patients, families, carers and communities who are affected by the disease, focuses its online display advertising on healthcare sites such as patient.co.uk.