John Cowan, SBG executive chairman, said: "We are very disappointed to lose Lisa and I tried very hard to keep her.
"However I understand her decision and at a time when the business is going through a lot of change it is perhaps not a surprise.
"She goes with our very best wishes."
Cowan said the Sesame board is due to meet tomorrow and will discuss who will take on Martin's responsibilities.
Martin was appointed into the Sesame Bankhall job earlier this month following the departure of John Cupis to head up mortgages at Openwork.
At the time Stephen Gazard was appointed managing director and now leads SBG's support for advisers across wealth and mortgages.
Martin has worked in the financial services industry for almost 35 years and is the latest member of Sesame Bankhall's senior mortgage team to leave the business.
Cupis joined the network in 2007 and had been responsible for leading the mortgage business until his departure at the start of the month.
In early 2014 former chief executive George Higginson also stepped down following a failed attempt to do a management buy out of the firm.
