In a statement issued by FleishmanHillard, the agency said it and Williams had "mutually agreed to separate".
Williams said. "I have had an absolutely fantastic time over this period. I am now really excited to be looking forward at new challenges and new opportunities."
Williams has more than 18 years' experience providing comms advice to political parties – having worked for the Labour Party in the run-up to the 1997, 2001 and 2005 elections – and to major international companies.
Previously, Williams was an associate director at GPC International before it merged into FleishmanHillard. He has also worked at Bell Pottinger, where he was a public affairs consultant.
During his tenure with the Labour Party, Williams was a senior political adviser and worked in the party’s campaign headquarters during the lead-up to its 1997 landslide victory.
He returned to work for the party in the 2001 and 2005 elections in Tony Blair’s office as part of his private office media team.