The agency will now provide communications planning support across the whole of HMRC.
Media activity will include education on new legislation and services, with a focus on improving engagement with the self-assessment and tax credit deadlines.
MG OMD replaces PHD on the account. HMRC is also believed to have invited Carat, MEC, MediaCom, Mindshare and UM London to compete for the business.
Katrina Bozicevich, the client business director at MG OMD, said: "We strongly believe that the diverse portfolio of successful campaigns run out of HM Revenue & Customs are of significant importance in the current economic environment."