Metroline comms manager Paul Schafe said the bulk of the agency's work would be to assist local level lobbying on issues such as planning, in its key operating area of London.
The agency has also been asked to promote the firm as an attractive employer in a bid to better recruit and retain staff. It will also advise on European transport issues.
The hire comes as London public transport issues gain greater prominence with the introduction next year of congestion charging.
The firm, which operates more than 80 routes across London, employs around 2,000 staff and also has services in Scotland.
Schafe said: 'London government is an area that we have highlighted as needing to be more involved in.'
AS Biss chairman Adele Biss said: 'One of our first tasks will be to communicate to key London opinion-formers that Metroline is the first London bus operator to meet the Mayor's expectation that all new bus drivers attain the BTEC qualification.'
Metroline is headed by MD Keith Moffat, to whom the agency reports.