He will be replaced by Scottish Widows Bank head of operations Gary McGregor, who will step up as managing director on an interim basis from 1 January.
Fleming (pictured) will take up the role of mortgage transformation director in January.
A statement from the lender said this would involve driving innovation across the wider group’s mortgage brands – which include Halifax, Lloyds Bank and BM Solutions as well as Scottish Widows Bank.
It added that Fleming had led Scottish Widows Bank through “an extremely successful two years”.
Lloyds Banking Group managing director of intermediaries and specialist brands Mike Jones, said: “We will continue to invest in and grow Scottish Widows Bank in 2018 as part of our ongoing commitment to the intermediary market.
“Lloyds Banking Group is counting on Martin to develop its broader mortgage business, and under Gary’s leadership, Scottish Widows Bank will maintain Martin’s great progress.”
Fleming added that he believed the team was in a strong position “to deliver the exciting plans we have for 2018”.
“I’ll be aiming to replicate some of Scottish Widows Bank’s successes across Lloyds Banking Group’s broader mortgage business in my new role and am confident that we are in great shape to do so.”