The 56-year-old, who will probably “end his days behind bars”, smothered the sedated Electra Brown writer and dragged her body into a cesspit last year.
She was found in the foetid burial site beneath their garage three months after she abruptly disappeared in April last year. Dead at her side was her pet dachshund Boris.
Stewart was convicted of the “despicable” killing, fuelled by greed for his bride-to-be’s riches, in a unanimous verdict at St Albans Crown Court.
Simon Russell Flint, defending, said: “He has burdened both the family of Helen Bailey and his own family with incalculable loss, a lifetime of misery, sorrow and suffering and he will have a long time to reflect on the consequences of those actions.
“The likelihood is, given his state of health, the sentence has a like effect of a whole-life order.
“There is every prospect and likelihood Mr Stewart will end his days behind bars.”
Diane Stewart, an epileptic who was mother to his two sons Jamie and Oliver, was found dead in the couple’s garden in June 2010. An inquest at the time concluded she died of natural causes from a “sudden unexpected death in epilepsy” and her body was cremated.
Detective Chief Inspector Jerome Kent, who helped secure Stewart’s conviction, said: “You will not be surprised that police investigating Ian Stewart for the murder of Helen Bailey would consider if there are any similar links to the death of his first wife.
“There is not a murder investigation into Diane Stewart, there is a re-examination of a sudden, unexpected death. It is only right that I would look back on somebody’s past.”
The former software engineer, wearing jeans, showed no emotion as he was found guilty by a jury of seven men and five women of murder, fraud, preventing a lawful burial and three counts of perverting the course of justice.
As he was led from the dock, he fixed his gaze on his eldest son, who did not meet his eye.
Outside the courtroom, tearful relatives and friends of 51-year-old Ms Bailey embraced. Her family welcomed the verdict, reached after five and a half hours, but said her death had left them in a “long shadow of loss”.
Judge Andrew Bright, who will sentence Stewart today, told Mr Russell Flint: “You will have a hard job to persuade me this wasn’t a murder for gain, it’s quite clear to me that it was.”
Stewart targeted the vulnerable widow on the internet in 2011, earning his way into her trust and later her £3.3 million estate after launching a “love-bombing” offensive.
Over many weeks, Stewart surreptitiously fed Ms Bailey his prescription anti-insomnia drug, Zopiclone, possibly by slipping it into her morning scrambled eggs.
A pillowcase found next to her body when it was discovered last July led the prosecution to suggest Stewart used a pillow to smother her while she was stupefied by the sedatives.