It follows a mobilisation phase which started in November last year when Mouchel began working alongside the Council's in-house team in readiness for the contract start. The contract award was announced in October.
The contract covers highways, environmental, transport and development professional services and is worth up to £5m annually depending on the extent of support required.
Subject to performance targets being achieved, the contract could be extended annually for up to a maximum overall term of 12 years.
Commission director Charles Ferrar said: "Norfolk is a brand new client for us and it's also a new geography for us. To ensure we're ready from day one we've worked closely with our client and the incoming works and traffic signals contractors, Lafarge Tarmac and Imtech, respectively. I'm delighted that our team is co-located with our client at County Hall in Norwich."
The Mouchel team now includes some staff who transferred from the previous service provider. Recruitment is well under way to fill a few remaining posts.
Norfolk's highways network includes 9,800km of local road, 4,100km of footways, 50,000 streetlights, 2,000 bridges and other structures, and 375 signal-controlled junctions and crossings.