Open Road will take over public affairs work for the electricity and gas network following nearly seven years with Bell Pottinger Public Affairs, which repitched for the work.
The brief starts next month and will last at least two years, with a possible 12-month extension.
The appointment comes as the energy industry strives for a voice in the debate on the Government's flagship Energy Bill, which is intended to drive £110bn of investment in new low-carbon power.
Graham McMillan, CEO of Open Road, said his firm would get involved at 'an absolutely critical time' for the future of energy in the UK.
McMillan said lobbying work would cut across Parliament, business stakeholders such as the British Chambers of Commerce, and consumers.
'If these reforms don't go through and these investments do not materialise, there could be an energy shortage and lights starting to flicker,' he added.
'Our work will be to help the National Grid, which is a supporter of the new framework, be a helpful provider of expertise and analysis on the market to help inform the debate.'
The Energy Bill, which plans to introduce a system of guaranteed long-term power price contracts, had its second reading in Parliament in December, but is not expected to become legislation until the end of next year.
National Grid head of corporate media relations Chris Mostyn said: 'After a thorough pitch process and some excellent submissions, we have chosen Open Road to support our UK public affairs work.
'We are looking forward to working with the agency on the challenges facing us and the energy industry.'
It is understood National Grid has yet to conclude the revamp of its £9m-a-year community engagement roster, revealed by PRWeek in September.