The year-long deal - understood to be a first in local government - will see Westminster PROs managing Richmond's comms and handling a Best Value review of communications, which will start in June.
This three-month review will be the first activity Westminster PROs will undertake in the deal, which has come under criticism from local Lib Dems at Conservative-controlled Richmond.
Concerns focus on the reported £80,000 cost to Richmond, including £20,000 for the review, and £60,000 for additional management support.
Details of the arrangement include the full-time posting of Westminster senior media officer Cormac Smith at Richmond. Also, Westminster comms head Alex Aiken will spend one day a week working for Richmond, with other PROs drafted across 'as and when necessary', according to Aiken.
Richmond's most senior PRO, communications manager Emma Anderson - who oversees a team that comprises just a senior comms officer and a part-time press officer - will report toAiken.
Aiken said the aim was to boost Richmond's media relations 'at a London level', in addition to assisting in the development of a new magazine for residents and boosting the councils' internal comms.
Discussions between the two councils began following the Comprehensive Performance Assessment - the council league tables published last December - which identified communications as an area in which Richmond should improve.
IPR Local Government Group chair Pat Gaudin welcomed the deal: 'There are already some areas where local government comms networks share best practice. This partnership is one step further'.