The agency has been created to "promote London with one single voice", and will be led by the interim chair, Dame Judith Mayhew-Jonas, chairman of the New West End Company.
The agency will have the responsibility of promoting the capital as a destination to visit, work, invest and study in, and has been provided with £14m worth of funding for the next four years.
The three bodies have been merged after the Coalition Government revealed it was cutting its £480m settlement to the London Development Agency (LDA) from this month. The LDA traditionally provided funds of £11.7m to VisitLondon each year.
Martine Ainsworth-Wells, marketing director for VisitLondon, is understood to be involved in the marketing department during the launch period of the new agency.
The identity of the marketing director for London & Partners is not yet known and is not expected to be announced today.
The board will also include Grant Hearn, executive chairman of Travelodge.
Last month, the mayor called a pitch for an agency to handle advertising work for the new body.