The decision to run a beauty parade with non-panel firms is a particular blow to Slaughters and Addleshaws, which could not be reached for comment.
Slaughters is Diageo's key corporate adviser and IT partner Nigel Swycher has been attempting to refocus his technology, media and telecoms practice in the buoyant outsourcing sector.
Diageo was also one of Theodore Goddard's biggest clients prior to its merger with Addleshaw Booth & Co, and it continues to be an important client.
The relationship stems from Theodore Goddard's links with Guinness, formed prior to the merger with Grand Metropolitan in 1997, which created Diageo. In former IBM in-house counsel Margaret Harvey, the firm also has plenty of outsourcing expertise.
Masons partner Shelagh Gaskill is leading the team advising on the seven-year agreement, which will see IBM take over and transform Diageo's IT infrastructure.
The instruction is a coup for Masons, which is likely to land further work as Diageo seeks to outsource all of its IT and other non-core business functions.
Masons' IT team is gaining a strong reputation in the outsourcing sector. This latest agreement follows billion-dollar deals for Boots and the Post Office.