Lewis has spent four years as head of the two organisations. They administer the Social Enterprise Investment Fund, which helps health organisations to become social enterprises, and Communitybuilders, a permanent endowment that lends to community organisations.
An SIB spokeswoman said Lewis did have another job to go on to but could not yet reveal what this was.
The Social Investment Business also ran the Futurebuilders loan fund, which gave out about £28m in grants and £127m in loans to help organisations bid for public service delivery contracts.
It was instrumental in setting up the welfare-to-work bidding consortium 3SC and recently announced plans to set up several new bidding consortia similar in size to 3SC, working in sectors such as children’s services, health, education, transport, leisure and offender management.
"The work of the two companies in helping community organisations become sustainable and impactful, and civil society organisations win and deliver public service contracts, is important both politically and socially, and I am very proud to have played a part in that," a statement from Lewis said.
Darren Garner, the finance director at the Social Investment Business, will serve as Lewis’s interim replacement until a permanent appointment is made.