The firms beat off competition from 16 other firms who tendered for the work.
The agency, which manages the property portfolio for the Ministry of Defence, was set up after the Government's property lawyers was disbanded over a year ago.
Eversheds will provide property advice in the central regions, Bond Pearce will cover the South West, Michelmores is acting for the Eastern area, while Richard Paddock & Partners will advise in the South East.
A Defence Agency spokes-man says the selection process began in September after nearly 60 firms expressed an interest in applying to tender.
The work involves providing property advice to the Defence Agency's sites in England and Wales.
Anne Hayward, property partner at Eversheds is heading up the work with Roger Nankervis, a property partner at the firm's Newcastle office.
Each of the firms has been contracted for five years to the Defence Agency, but the contract also includes a two-year extension period.