Pavey has taken the job after 15 years at the agency, where she was on the management board.
She will start as head of UK corporate communications at the end of this month, stepping in for Tom Stevenson, who has become investment director for Fidelity Personal Investing.
The role, which reports to head of business development Ed Dymott, will include oversight of Fidelity Personal Investing, DC & Workplace Savings and FundsNetwork.
Finance specialist Pavey said she had "always admired" Fidelity, which manages funds worth more than $270bn for private individuals and institutions.
She called it "one of the most interesting and ambitious businesses in financial services right now, positioned right at the heart of the UK savings and retirement market".
Pavey added: "I have had fifteen fantastic years at Lansons, working alongside and learning from the best communicators in financial services. I am looking forward to applying all that experience in my new role."
Earlier this week, PRWeek revealed that Lansons had hired Clare Anderson as it looks to expand its senior level corporate offer.