Chambers' four-year secondment from the Department for Business, Enterprise and Regulatory Reform to the postal market regulator ends in September.
Brown joined Postcomm at the beginning of 2008 as director of market development, having worked previously at DHL Express UK and Royal Mail.
At Royal Mail he was sales and marketing director of the Parcelforce division.
Nigel Stapleton, chairman of Postcomm, said: "Tim's extensive experience of the sector, gained at Royal Mail and in the wider competitive market, will be invaluable as we work towards establishing a new regulatory framework that will secure the universal service and provide more choice for all mail users and providers."