Sabrina Kouider, 35, and Ouissem Medouni, 40, built a warped fantasy around music mogul Mark Walton and accused timid 21-year-old Sophie Lionnet of being in league with him.
Kouider collapsed in tears and Medouni hung his head as the jury foreman returned the verdicts at the Old Bailey following a lengthy trial and almost 30 hours of deliberations.
Miss Lionnet’s mother, Catherine Devallonne, also wept as Judge Nicholas Hilliard QC said he was sure the allegations against her daughter contained “no truth whatsoever”.
Describing the killers as “monsters”, Mrs Devallonne said: “These self-obsessed individuals who murdered Sophie did not believe Sophie had a value.
“Those monsters repeatedly beat Sophie. They starved, tortured and broke her until she could no longer fight.
“They took away her dignity and finally her life painfully ebbed away until Sophie struggled to take her final terrified breath in the bath.
“Our Sophie will soon be laid to rest. No god will ever forgive you both for what you have done to our daughter.”
The victim’s father, Patrick Lionnet, said: “Sabrina and Ouissem have not only stolen the life of my daughter so brutally and without remorse, they have also stolen mine.”
He said what the couple did to his shy and reserved daughter was “beyond comprehension” and “unforgivable”.
In the weeks leading up to her death in September last year, the couple beat, starved and tortured the shy au pair by dunking her head into water until she confessed.
Having killed her in the bath, they then threw her body on a bonfire in the garden of their home near Wimbledon, south-west London, as they barbecued chicken nearby.
When firefighters were alerted by neighbours to pungent-smelling smoke, Medouni tried to pass off the charred remains as a sheep.
Kouider claimed to police that Miss Lionnet had run off with Mr Walton in an attempt to frame him for her disappearance.
The defendants later admitted disposing of Miss Lionnet’s body but denied her murder, blaming each other for her death.
The jury found both of them guilty of murder following a two-month trial which was described as stranger than fiction.
Aisling Hosein, from the Crown Prosecution Service, said: “Only Kouider and Medouni know exactly how they killed Sophie but the prosecution was able to prove that she died as a result of purposeful and sustained violence, and not by accident.
“They were both jointly involved and came up with a plan to try and destroy her body and escape responsibility for this horrendous crime.”