Pfizer issued a tender in 2013 and CWT Meetings & Events won the account following a competitive pitch.
Pfizer previously worked with Zibrant, which was appointed in 2011 to deliver congress management work, including delegate management and project management.
The contract represents a major win for CWT, which earlier this year reported a 20% increase in revenue for 2013, representing the third consecutive year that it has seen double-digit growth.
It has also strengthened its live events and sales teams with several new hires, and launched into China in April following the acquisition of a tour operating licence from the China National Tourism Administration (CNTA).
Pfizer declined to comment on the appointment.