Lambkin joined Diageo in January 2014, taking over from Ian Wright.
A Diageo spokesperson said: "After three good years, Charlotte has decided to leave the business and everyone at Diageo wishes her well. She leaves a corporate relations function well set with a strong team in place. Her successor will be announced in due course."
The company did not comment on when Lambkin will be leaving her role.
Before joining Diageo, Lambkin spent more than nine years as group communications director at BAE Systems.
Last year Vicky Sheriff left her role as global comms director at Diageo, moving to Heathrow Airport and more recently to Which?
In September, PRWeek reported that Diageo, which owns drinks brands including Guinness, Smirnoff, Baileys and Gordon's, had given its global corporate comms account to Teneo Blue Rubicon.