Barclays said Lucas and general counsel Mark Harding will leave once successors have been found. The bank said in a statement that given the men's seniority, that could take "a considerable time."
Lucas did not disclose reasons for his departure, but said his six-year stint as group finance director had occurred during "the most eventful period during which anyone could have occupied a role such as mine."
Lucas is one of several past and present Barclays staff being investigated over whether the bank broke the rules when it took big cash infusions from Qatar's sovereign wealth fund in 2008.
He also was one of the most senior survivors of a period that saw several top executives, including CEO Bob Diamond, leave since a rate-fixing scandal erupted last year. Barclays was fined $453 million by U.S. and British authorities after it emerged that executives had been involved in a campaign to rig a key interest rate known as LIBOR.
Barclays also is one of several British banks involved in a scandal over the mis-selling of payment protection insurance, in which consumers were signed up for inappropriate and expensive insurance products. That scandal has already cost Barclays hundreds of millions of pounds in compensation.
Last week CEO Antony Jenkins said he was forgoing his annual bonus in light of the "multiple issues of our own making besetting the bank."