The TV cook said the jury was "faced with a ridiculous sideshow of false allegations" about drug use which made focus on the actual criminal trial impossible.
Francesca Grillo, 35, and her sister, Elisabetta, 41, were found not guilty of one count each of defrauding Ms Lawson and her ex-husband, Charles Saatchi.
Ms Lawson admitted during the three-week trial that she had taken cocaine, after allegations of drug use arose as part of the defence case.
But Scotland Yard said today that it would not be investigating unless new evidence comes to light.
Ms Lawson said in a statement: "When false claims about habitual drug use were introduced, I did everything possible to ensure the CPS (Crown Prosecution Service) was aware of the sustained background campaign deliberately designed to destroy my reputation."
During the trial she had accused multi-millionaire art dealer Mr Saatchi of threatening to "destroy" her.
She said that, despite doing her civic duty, she was "maliciously vilified without the right to respond".
The Italian sisters had been accused of using company credit cards, spending hundreds of thousands of pounds on designer goods for themselves while working as personal assistants to the celebrity couple.
They claimed every purchase had been approved by their then-bosses.
The trial could have collapsed after Prime Minister David Cameron declared his support for "Team Nigella" in a magazine interview halfway through the trial - his comments prompting the defence calling for the trial to be brought to an end.
After a day of drama and tension at Isleworth Crown Court in west London, during which Elisabetta collapsed upon arrival, the younger sister was delighted to learn of the not-guilty verdicts.
Neither woman had been in Court 8 when the jury foreman read out the verdicts, the seven men and five women having been deliberating for almost nine hours.
The pair had been excused from the dock because Elisabetta stopped breathing following a panic attack yesterday and spent two hours in hospital last night.
Francesca's barrister, Karina Arden, was seen in the court foyer after the verdicts, proclaiming to her client: "C'e un Dio!" - Italian for "There is a God".
Francesca replied with a smile: "C'e un Dio!" and the pair embraced.
During the trial, Ms Arden said that, like Ms Lawson, she had a "penchant" for all things Italian.
Anthony Metzer QC, representing Elisabetta, said his client was "relieved" and "crying her eyes out".
Mr Metzer and Ms Arden turned to each other a few moments after the verdicts were delivered and smiled.
When the judge left the court, they hugged members of the defence team, one of whom was crying what appeared to be tears of joy and relief.
It was alleged that between January 1 2008, and December 31 2012, the two women committed fraud by abusing their positions as PAs, using a company credit card for personal gain - and were accused of spending more than £685,000 on themselves.
Elisabetta, sometimes referred to in court as Lisa, and Francesca, both of Kensington Gardens Square, Bayswater, west London, had been accused of living the "high life".
In a sensational twist, their defence lawyers introduced allegations of drug-taking by Ms Lawson and marital strife involving the celebrity couple.
It was claimed by the defence that there was a culture of secrecy within the high-profile couple's marriage and that the Grillo sisters were aware of Ms Lawson's alleged drug use, while Mr Saatchi was not.
The defence claimed that Elisabetta's knowledge of Ms Lawson's supposed drug use materially affected the TV cook's attitude towards her spending.
During the trial the jury heard details of an email sent by Mr Saatchi in which he accused Ms Lawson of being off her head on drugs and branding her "Higella".
He said in that message: "Of course now the Grillos will get off on the basis that you... were so off your heads on drugs that you allowed the sisters to spend whatever they liked and, yes, I believe every word the Grillos have said, who after all only stole money."
Giving his evidence, he said it was a "terrible, terrible mistake" for that email to become public.
Asked if he believed Ms Lawson to have allowed staff to spend what they liked because she was under the influence of drugs, he replied: "Not for a second."
Jurors were told that Elisabetta had not initially planned to use her former boss's alleged drug-taking in her defence in an effort to protect the TV cook.
An original defence case statement for Elisabetta from August did not include allegations of Ms Lawson's drug use because she did not want them raised in court as she felt sympathy for her, jurors heard.
But an extra statement added in November did include the claims.
The additional statement, read to the court by Elisabetta's barrister, Mr Metzer, said his client would assert that Ms Lawson "habitually indulged in the use of Class A and Class B drugs in addition to the abuse of prescription drugs" throughout the PA's employment.
It went on: "This evidence is of substantial importance as it explains why Ms Lawson initially consented, or appeared to consent, to the expenditure as the defendants were intimately connected to her private life and were aware of the drug use which she wanted to keep from her then-husband Charles Saatchi."
Ms Lawson admitted during the trial that she took cocaine with her late husband, John Diamond, when he found out he had terminal cancer, and on another occasion in July 2010 during her troubled marriage to Mr Saatchi.
But the 53-year-old, who also admitted to smoking cannabis, told the court the idea that she is a "drug addict or habitual user of cocaine is absolutely ridiculous".
She described Mr Saatchi as a "brilliant, but brutal man" who subjected her to "intimate terrorism".
The food writer claimed her 10-year marriage to Mr Saatchi became so unhappy it drove her to drugs, which made an "intolerable situation tolerable".
In her evidence, Francesca said she "frequently" found rolled up banknotes with white powder on them in Ms Lawson's handbags.
Mr Saatchi was accused of using the trial against the Grillo sisters to "attack" his former wife, from whom he was divorced earlier this year.
In her evidence, Ms Lawson accused Mr Saatchi of threatening to "destroy" her.
Mr Metzer, said Elisabetta had been caught up in the "crossfire" between the former couple.
The Grillo sisters' solicitor, Richard Cannon, said outside court that his clients were "naturally relieved" at the verdicts after a "long, hard fight played out in the gaze of the world's media".
There was a frenzied media scrum as the sisters left the court nearly three hours after being cleared.
Accompanied by security staff, they were guided out of the building through the crowds of photographers and camera crews, into a waiting car with blacked-out windows, without making any comment.