Bridget Kathleen Gilderdale, known as Kay, had already admitted aiding and abetting the suicide of 31-year-old Lynn Gilderdale at the family bungalow.
She was given a 12-month conditional discharge for assisting the suicide.
The jury returned its verdict after deliberating for less than two hours.
Cries were heard from the public gallery as the not guilty verdict was read by the foreman.
Mr Justice Bean said: "I do not normally comment on the verdicts of juries but in this case their decision, if I may say so, shows that common sense, decency and humanity which makes jury trials so important in a case of this kind.
"There is no dispute that you were a caring and loving mother and that you considered that you were acting in the best interests of your daughter."
The judge added that she acted selflessly and with exemplary devotion for 17 years.
Jurors heard she crushed up pills and fed them through her daughter's nasal tube, handed her morphine and injected three syringes of air into her vein after she made a failed suicide bid.
Gilderdale, 55, initially tried to stop her daughter ending her life following a 17-year battle with the chronic fatigue illness ME but backed down after she said: "I want the pain to go."
Lewes Crown Court was told she was a loving and devoted mother to her daughter, who was struck down by ME at the age of 14 and needed round-the-clock care at her home in Stonegate, near Heathfield, East Sussex.
Miss Gilderdale, a once active, sporty and musical girl, led an "unimaginably wretched" life in her later years.
She was paralysed from the waist down, unable to speak, eat or drink and was fed through a tube.
Communication to her parents, who were divorced but remained supportive of her, was through a form of sign language they devised themselves.
She was bed-bound, socially isolated, unable to sit up and developed suicidal thoughts, which she published on an online forum for people suffering from illnesses.
She had attempted suicide in the past, had drafted a "living will", placed a Do Not Resuscitate note on her medical records and considered ending her life at Dignitas, the Swiss-based assisted suicide clinic.
In the early hours of December 3 2008, she took an overdose of morphine by injecting the pain-relieving medicine directly into her vein, according to Gilderdale's own account to the family GP, Dr Jane Woodgate, on the morning of the death.
When she realised that the dosage was not high enough, Miss Gilderdale called out to her mother, who then spent around an hour trying to persuade her not to press ahead with killing herself.
But after telling her mother that she wanted the "pain to go" and that she did not want to go on, she set about, over the course of 30 hours, helping her end her life, the trial heard.
Gilderdale handed her two syringes of morphine, consisting of 210mg each, which her daughter administered herself through her Hickman line directly into her vein.
Hours later, at about 6am, Gilderdale felt that the morphine had not achieved her daughter's aim of killing herself and searched the house for tablets, fearing that she would be brain-damaged.
These pills were crushed using a pestle and mortar and then inserted into her daughter's nasogastric tube.
Evidence from Sussex Police found that during the period until her death, Gilderdale trawled the internet for details on drug overdoses and euthanasia.
Following "two or three" more doses of morphine, three syringes of air were pumped into her Hickman line with the intention of causing air embolisms, it was alleged.
Miss Gilderdale died at 7.10am on December 4, with only her mother present. A post-mortem examination at the Conquest Hospital in Hastings found the cause of death was morphine toxicity.
Gilderdale's ex-husband, Richard Gilderdale, told the court he discovered his daughter had died after he received a text message from his former wife, saying: "Please can you come now. Be careful. Don't rush."
When he received the message, he knew what had happened, he told the court.
He said he could never imagine his ex-wife trying to kill their daughter, saying she gave up work without regret and rarely took a holiday.
Describing Gilderdale's reaction to the death, he said: "Her world had come to an end, she was crestfallen, heartbroken - everything you could describe about somebody who had been to hell and back."
Miss Gilderdale had attempted to kill herself before in mid-2007 with a morphine overdose, but her father walked into her bedroom to find her sleepy.
Retired Sussex Police officer Mr Gilderdale said Gilderdale, whom he divorced in around 2001, remained positive that she would get better, although he privately thought otherwise.
Despite their marriage split, they remained actively supportive of their daughter.
Throughout the trial, Gilderdale was supported by a large number of family and friends, including her son Steve and her former husband.
She declined to give evidence. Jurors were told that she could not be tried for murder as it was uncertain whether her daughter died from the overdose she gave herself or from that given by her mother.
Standing on the steps of the court, Gilderdale's son, Steve, read a statement praising the verdict.
Flanked by his mother and father, he said: "We believe this not guilty verdict properly reflects the selfless actions my mother took on finding that Lynn had decided to take her own life, to make her daughter's final moments as peaceful and painless as possible.
"These actions exhibit the same qualities of dedication, love and care that mum demonstrated throughout the 17 years of Lynn's illness.
"I'm very proud of her and I hope she will be afforded the peace that she deserves to rebuild her life and finally grieve for the death of her daughter."
The case has drawn parallels with that of Frances Inglis, the mother jailed at the Old Bailey last week for injecting her brain-damaged son with a lethal dose of heroin, and reignited the debate about assisted suicide.
Mr Justice Bean asked prosecutor Sally Howes QC to explain "why it was considered to be in the public interest" to pursue Gilderdale on the attempted murder charge when she had already admitted aiding and abetting her daughter's suicide.
Ms Howes said the prosecution decided at "the highest level" to press on with a jury trial after Gilderdale had told her GP and the police that she had administered an air embolism with the intent to end her daughter's life.
"There was no submission of no case to answer and the matter went to the jury," Ms Howes said.
Defence counsel John Price QC asked the judge to consider an absolute discharge based on interim guidelines by the Director of Public Prosecutions, Keir Starmer QC, who last year outlined 16 "public interest factors" in favour of a prosecution and 13 factors against taking legal action to bring clarity to existing assisted suicide legislation.
The move came after law lords backed multiple sclerosis sufferer Debbie Purdy's call for a policy statement on whether people who help someone commit suicide should be prosecuted.
Mr Price said that if the DPP was handling the case today, in light of the verdict, it would not be considered in the public interest to prosecute.
