Bailey, 51, went missing in April and Stewart, 56, was arrested for her murder after her remains were discovered, along with dachshund Boris, at their Hertfordshire home.
He was convicted after a trial at St Albans Crown Court, in which prosecutors said he plotted to murder his partner to get her money.
The court heard he had plied her with sleeping medication for months before killing her.
Prosecutor Stuart Trimmer told the trial: “Once Helen Bailey was effectively sedated, suffocation would have been quite simple, enabling the defendant to kill her with little or no resistance.”
Stewart was also convicted of fraud, three counts of perverting the course of justice and preventing a lawful burial of a body.
Detective Chief Inspector Jerome Kent said: “To kill somebody was despicable enough, but to dispose of her in the way he did and lie to everyone including his own children shows how wicked and despicable that man is.”
Police have also said they will reexamine the death of Stewart’s first wife Diane in 2010.
She died after an epileptic seizure in the garden of their home in Cambridgeshire.
During the three months Bailey was missing, Stewart sparked a major search effort after feeding her loved ones the lie that she had left abruptly to seek some “space” at her seaside cottage in Broadstairs, Kent.
He also paid for flyers and attended a dog walk as part of the search effort for her, the trial heard.
During earlier police searches of the property, Stewart concealed the cesspit he had hidden her body in by blocking its opening by parking a car on it.
Bailey’s mother Eileen Bailey told the trial she was becoming increasingly concerned about her daughter’s deteriorating state of mind and repeated forgetful incidents before the murder.
In the weeks before Bailey vanished, she confided in her mother about inadvertently leaving her dog on the beach, taking an item scanner from a supermarket and not being able to recognise her hands on a computer keyboard.
Speaking to the court via video-link, with a framed picture of the author visible behind her shoulder, Mrs Bailey told the court: “That really worried me.”
She added: “She just had such a good memory beforehand.”
A week before her alleged murder, the 51-year-old phoned her mother, deeply worried after falling asleep for five hours - despite having a full night’s rest.
Asked about her views on the couple’s relationship, Eileen Bailey said: “Well, I felt uneasy about it, latterly I was quite unhappy - mainly because of Helen’s state of mind.”
After the verdict, Bailey’s brother John Bailey said: “Despite this victory for justice there can be no celebration. Our families have been devastated and nothing can ever bring Helen back to us, or truly right this wrong.
“A long shadow of loss has been cast over the lives of so many who will always remember Helen with enduring love and affection.”