Miller, who joins from Starcom where she was the digital director, previously worked at i-level as the client director on Orange.
The appointment sees her reunited with the Maxus digital strategy director, Tom Dunn, who also previously worked at i-level.
She will be responsible for maximising digital revenue, delivering best digital practice for clients and helping to drive new business.
Miller will report to the Maxus chief executive, Lindsay Weedon, and will work across all the agency's clients including BT, Mercedes-Benz and Fiat.
Starcom is understood to be actively recruiting to find Miller's replacement.
Weedon said: "I'm delighted that Kirsten is joining us as we continue to build momentum. She is fantastically smart, digitally savvy and I know clients will love her."