Nationwide Building Society is to close its branches in Hampstead, Golders Green and Cricklewood.
Colette McNeil said she was "shocked and distressed" to receive a letter informing her of the closure of branch in Heath Street.
In the letter, seen by the Ham&High, the mutual bank wrote to say "we're sorry to say we're closing three of your local branches".
On August 18 at 2pm, the Cricklewood, Golders Green and Hampstead branches will close.
The nearest alternative branches are in Camden Town, Kilburn, Holloway, Finchley and Wood Green.
Colette said: "I am lucky to be fairly fit, but have been in the branch with many older people who are not so mobile, and for whom a journey to another branch and then to have to wait in a queue would be out of the question.
"Also, those of us of a certain age are unwilling or unable to use online banking mainly because we mistrust the technology and the security of the systems.
"Many do not even have a device to enable them to do so, and are completely dependent on having the staff to offer help and guidance."
She said customers value their relationship with staff and said if the branch is to close, "people will have to go to Camden, a trip on the underground, or Kilburn, involving two buses which will be difficult for elderly customers".
Hampstead's Conservative councillor Stephen Stark (Con, Hampstead Town) said: "It's not good for residents and it's not good for businesses."
Hampstead and Golders Green are already set to lose HSBC branches in September,.
Lloyds Bank on Rosslyn Hill shut in 2019 and Nat West in 2017.
"It puts more pressure on Barclays Bank and the Post Office. If we lose those it will be catastrophic for Hampstead," Cllr Stark added.
A Nationwide spokesperson said the company is not closing "vast swathes" of branches but "must consider whether it is the interest of our wider membership to keep them open".
"We need to determine whether it makes more sense to use that money to improve and retain branches elsewhere in our network, particularly in areas where there is lower provision," they said.
They added "up to 60% of Hampstead branch members are using other branches, and only 1% of Hampstead members are using their respective branches exclusively and frequently."
Members needing support should contact their branch.
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