Finsbury has replaced independent consultant Tony McGarahan, who has advised Equitable's board for the past three years.
McGarahan said he had always intended to leave Equitable after the court case had finished. Closing submissions in the action are scheduled for December.
McGarahan would not say whether he planned to take on an in-house or agency role or continue with his consultancy firm, Heath Corporate Communications.
Policyholders have called for the resignation of Equitable CEO Charles Thompson and chairman Vanni Treves after the mutual's negligence case became untenable and its claims reduced to £650m.
Equitable hopes selling the business or restructuring its fund could persuade policyholders to reprieve Thompson and Treves.