Cllr Claire Hickson, Labour councillor in Chaucer ward, has been appointed to the borough's cabinet.
The announcement was made by council leader Peter John at last week's annual council meeting.
Cllr Hickson, who was elected to the council two year ago, is the only Labour councillor in the five wards that make up Borough, Bankside and Bermondsey. She joins a cabinet that is dominated by councillors representing the central part of the borough.
She takes over the communities portfolio from Cllr Abdul Mohamed who is now the borough's deputy mayor.
"I am delighted to welcome to the cabinet Chaucer ward's Cllr Claire Hickson as cabinet member for communities and economic development," said Cllr John.
"Claire will oversee the continuing and important work we need to do engaging with our communities. Since last year's disturbances the work this council has done through our community engagement team has been exemplary and we should all take pride in that fact.
"Claire will also take on responsibility for economic development over the next six months whilst Cllr [Fiona] Colley remains on maternity leave.
"With rising unemployment, and particularly rising youth unemployment, in the borough it is vital that we place economic stimulation at the heart of the work which we do, and I know Claire will deliver on that for us.
Cllr Hickson explained her new responsibilities to the London SE1 website after last week's meeting: "On the communities side I'll be looking at how we are supporting all kinds of community organisations, from tenants' and residents' associations to business associations, and what kind of support we give them.
"I'll also be working on the economic development side, which is about developing a strategy looking at how we are working with businesses. We've got a big challenge this year because [the Government] is going to localise business rates, so we really need to work on that relationship with business."
In other changes to the borough's cabinet, Cllr Dora Dixon-Fyle takes over the children's services portfolio from Cllr Catherine McDonald, who in turn replaces Cllr Dixon-Fyle as cabinet member for health and adult social care.
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