The LSDC was established in 2002 to promote sustainable development in the capital.
Arc Seven was launched last August by Alistair Clay, who was formerly media relations manager at Plan UK and is an ex-Daily Mirror reporter.
The Arc Seven team will lead the media relations strategy for the LSDC's 'London Leaders' project, which sees 15 business and community leaders and entrepreneurs across the city create and run sustainable ventures and initiatives.
Current London Leaders include minicab firm Addison Lee's CSR and environmental manager Lee Wickens, and Charles O'Malley, head of Europe for CSR firm Account-Ability.
Clay said: 'The London Sustainable Development Commission is doing some truly innovative work to ensure the capital is seen as a world leader in the sustainable development field.
'We look forward to putting this year's London Leaders in the spotlight to celebrate their fantastic work and to help thousands of Londoners lead more sustainable lives.'
LSDC chair John Plowman said Arc Seven had impressed with its 'understanding of sustainability issues and focus on delivering real outcomes'.