Karen Evans, 36, had crossed at high speed from lane two of the A55 in North Wales to a slip road but was unable to control her car, which somersaulted into the air.
The devastating crash killed her autistic son Jordan instantly and left Ms Evans and her daughters, aged three and 18, with serious injuries, Mold Crown Court heard.
Prosecutors said the accident happened because Ms Evans was "in a rush" but she said her steering wheel was at fault.
Mrs Evans sobbed with her face in her hands as the jury of seven men and five women returned their unanimous verdict after just over an hour and 15 minutes of deliberations.
Her family and supporters in the public gallery shouted: "Yes."
Before the trial began on Monday, Judge Peter Heywood had questioned whether, given the mother's loss, a trial in which she would have to re-live the awful events of her son's death was in the public interest.
He said: "Whatever the outcome of the trial, this is going to cause her considerable grief for the rest of her life.
"She is going to have to live with the consequences for the rest of her life.
"I'm simply expressing views of, perhaps, common humanity as a father myself.
"It must be a decision for the prosecution (but) I question whether this is in the public interest."
The Crown Prosecution Service responded by saying the case was in the "wider public interest".
After releasing Mrs Evans from the dock today, the judge told the jurors he "entirely agreed" with their decision.
He said: "It has been an awful ordeal for her - having lost a son and then having to go through a trial."
The crash happened on a slip road into services off the A55 near Northop on April 29 last year.
Jordan, who was autistic, had not wanted to go to school that day and deliberately missed his bus.
Mrs Evans decided to drive him to school and set off, with his sisters, from their home in Mancot shortly after 9am.
Levi-Marie, 18, who was being taken to college, was sitting in the front passenger seat with Jordan behind and three-year-old Lilly-May in a child seat next to him.
Prosecutor Oliver King claimed Mrs Evans was "in a rush and needed petrol" when she decided "too fast and too late" to enter the slip road.
But Mrs Evans told the court there was a problem with the steering wheel which led to the "split-second" decision to exit the dual-carriageway at high speed.
She denied she had been rushing and said she was unaware the petrol light had been on.
As her Vauxhall Corsa entered the slip road across hatched road markings, Mrs Evans was unable to control the vehicle and it somersaulted into the air.
The mother and daughters survived the collision but Jordan, a pupil at St Richard Gwyn High, in Flint, suffered massive head injuries and was killed instantly.
A post-mortem examination later concluded he died of a blunt force trauma to the head.
Forensic examinations of the car later found that a bolt in the steering wheel was loose.
"I just knew if I got the car off (the road) I wouldn't hit the central reservation or the other cars, killing all my children or somebody else's," she told the jury.
Outside court, Mrs Evans continued crying as she was comforted by a number of relatives, including daughter Levi, who told the jury she had no concerns about her mother's driving that day.
The teenager gave police a statement recalling how the car started veering from left to right and shaking just before her mother moved into the slip road.
The family left the building without making any comment.
In a statement, released by her barrister, Paul Smith, Mrs Evans said: "This has been a year of immeasurable pain.
"I have had to nurture my daughters through their recovery from the accident, to recover myself and to endure this prosecution.
"All while I have wrestled with the loss of my son and my part as the driver. I am grateful to my family and those close to me who have supported and sustained me.
"I bear no grudge against anyone and can understand why the accident in which my son Jordan died was fully investigated, but I have never understood why the Crown Prosecution Service (CPS) persisted with this prosecution.
"They knew from the police expert that there was a fault in my car's steering and they knew that the witnesses saw and heard my car behaving in an abnormal manner immediately before I tried to get off the dual carriageway.
"I particularly do not understand why they chose to ignore what my daughter, who was in the car, had told police within hours of the accident when she was in hospital.
"She had even made a statement to police.
"It has felt as if, the more they were asked by judges, out of common humanity, to review the case and reconsider their decision to pursue the prosecution, the more blinkered and entrenched the CPS became.
"I hope they will now have the courage to review they way they dealt with my case so that nobody in my position is ever put through this dreadful ordeal again.
"I asked the jury to find me not guilty on the evidence, not out of sympathy. Today, they have done just that and I am grateful.
"It is now time for me and my family to move on and grieve for the loss of Jordan."
Nick Price, Chief Crown Prosecutor for North Wales CPS, said: "This has been a very difficult and sensitive case to deal with and we fully respect the verdict reached by the jury.
"We recognise that Karen Evans lost her son as a result of the incident and we appreciate the dreadful impact this has had on both her and the rest of Jordan's family.
"We have considered this case very carefully from the outset in the light of those circumstances.
"However, we believe that the standard of driving demonstrated that day by Karen Evans warranted a prosecution in accordance with CPS national policy for prosecuting cases of bad driving.
"We did not take that decision lightly and in doing so we very carefully considered both the evidence in the case and the wider public interest in bringing a prosecution."
