Hannah Crabtree was previously comms manager at the Institute of Cancer Research and has also worked at Action Aid.
At Macmillan, she reports to director of communications Mark Silverside and replaces Liz North, who left a year ago.
Both the fundraising and campaigns teams are five-strong. Head of campaigns Anna Brosnan joined three months ago from the Department of Health where she was press officer.
Macmillan merged with the smaller specialist cancer information charity Cancerbackup in April. It has inherited one press officer from the charity and the rest have moved into the public affairs team.