He is based in London but will work with the company's staff across Europe, the US and APAC.
St Andrew started working on the festivals in June 2013, according to LinkedIn. Prior to this he had worked as a freelancer, for the agency KLPR and in magazine publishing.
Unruly, which has been owned by Rupert Murdoch's News Corporation since 2015, recruited new CEO Norm Johnston earlier this year; St Andrew said it was "great to be joining the company in this exciting new chapter".
The comms director role was previously held by former journalist David Waterhouse, who had spent more than seven years at the company.
There is no immediate successor to St Andrew's role. The Cannes Lions' press team this year consists of Camilla Lambert, Marian Branelly and Isobel Diamond, all of whom have worked on the festival for a number of years.
The festival has confirmed that it will not work with Propeller at this year's festival; the PR firm was given a media relations brief last year.
Cannes Lions 2018 runs from 18 to 22 June. The jury for the PR Lions, which will be awarded on the festival's penultimate day, was announced last week.