Susan Norman, 68, died when tonnes of mud and rubble engulfed her flat in Looe, Cornwall, in March 2013.
An inquest jury concluded Cornwall Council had not listened to "consistent and frustrated complaints" from residents about previous landslips.
The authority has apologised to Mrs Norman's family.
More on the landslide inquest and other news from Devon and Cornwall
The court heard neighbours had warned Cornwall Council about the risk of erosion and questioned the stability of the land years before the fatal accident.
Mrs Norman's landlady, Christina Miller, told the inquest as soon as her tenant raised concerns she had called in a structural engineer who warned about the risk of "catastrophic collapse".
A post-mortem report read to the jury at Truro Magistrates' Court said Mrs Norman died immediately.
The jury said in its findings: "Cornwall Council did not listen to regular consistent and frustrated complaints by the residents. Cornwall Council did not respond to an independent report highlighting risk."
It added: "The landslide was caused, or more than normally contributed to, by a failure of Cornwall Council to prevent the surface water moving from St Martin's Road on to the rear of Veronica's (Mrs Norman's house)."
After the inquest, Mrs Norman's daughter Helen Hazeltine said: "She was taken away from us and she didn't have to be.
"This should never happen again and I don't want anyone else to go through this, at all, ever."
Kate Kennally, Cornwall Council's chief executive, said: "I would like to apologise to Mrs Norman's family and friends and express our sincere condolences for their loss."
She said it would "not be appropriate to comment in more detail", as court proceedings against the council and its civil engineering arm CORMAC had been lodged.
A council ought to have known a grandmother faced "a real and immediate risk of death" from landslides, jurors have said.