Rhodes, who spent five years at Proximity in charge of media and channel strategy, will be responsible for DMS's media and data planning.
She will work alongside head of planning Steven Dodds, whose title also changes to joint head of planning, and will report to DMS managing director Elly Woolston.
Woolston said: "Diana is an outstanding practitioner and it will be great to be working with her again."
Rhodes' appointment coincides with an account win. The agency has won the DM business of children's hospice charity Chase, seeing off competition from Whitewater and TW Cat in a pitch.
Guildford-based Chase supports families of children with terminal illnesses. DMS won the brief on the strength of a creative concept designed to attract new donors.
The agency's direct marketing activity begins this month with a mailshot aimed at the charity's existing supporters.
This will be followed in September with a mailing aimed at non-supporters. Activity will include direct mail, door drops, radio and digital.
Woolston said: "It will be an exciting campaign, encapsulating warm, cold and reactivation activity, and we look forward to helping Chase attract new donors to support its future ambitions."