Lucas and general counsel Mark Harding will leave the company once once successors have been found, the bank said in a statement.
Given the men's seniority, that could take "a considerable time."
Lucas joined the Barclays board in 2007
Barclays has also seen several top executives, including CEO Bob Diamond, leave since a rate-fixing scandal erupted last year.
The bank was hit with a 453 million US dollars (£289 million) fine after it emerged that executives had been involved in a campaign to rig a key interest Libor rate.
Tonight, Lucas said: "It has been my great pleasure and privilege to serve as group finance director of Barclays for nearly six years now.
"While it has undoubtedly been the most eventful period during which anyone could have occupied a role such as mine, I have thoroughly enjoyed the challenge."
Harding added: "I shall be sad to say goodbye to Barclays when the time comes but the bank, and my colleagues, will always have my support and best wishes."
CEO Antony Jenkins said: "Chris and Mark both expressed to me late last year that they were considering stepping down from their roles at Barclays.
"The rationale which each shared with me was consistent and, typically, grounded in wanting to do what is best for the bank.
"Their decision to retire was theirs alone.
"Barclays is at an inflection point in our journey towards becoming the 'Go-To' Bank.
"We will shortly unveil the outcome of our strategic review, to which they have both made a major contribution.
"The execution of our change programme will take place over the next five to 10 years, and both Chris and Mark feel that now is the right time for them, personally and professionally, to pass the baton on in their respective roles to executives who can commit to seeing that programme to completion.
"Chris and Mark have given tremendous service to Barclays.
"The chairman, the board and all my executive committee colleagues and I shall be very sorry to see them go.
"I know they join me in recognising the quality of their contributions, the sacrifices they have both made in what have often been challenging times, and their unfailing collegiality.
"We will continue to rely on them very greatly in the coming months."