Angela Taylor and Paul Cannon shared a "venomous hatred" for William Taylor, who had steadfastly refused to grant his wife of 20 years a divorce before vanishing just before his 70th birthday last June.
His skeletal remains were found waist-deep in mud by a passing fisherman on a secluded riverbank near one of his farms in Gosmore, Hertfordshire, eight months later.
The landowner's body was so decomposed that determining a cause of death was impossible, experts told St Albans Crown Court.
Taylor, 53, and digger driver Cannon, 54, were jointly convicted of his murder by a jury of eight women and four men on Monday after little more than 11 hours of deliberations.
Grey-haired and burly Cannon, wearing a dark suit, shook his head as the verdicts were returned, while Taylor appeared to make no reaction.
Despite a lack of forensic evidence, jurors convicted the pair of murder after hearing over the course of the two-month trial dozens of violent WhatsApp messages expressing a desire to seriously harm Mr Taylor.
Mr Taylor, known as Bill, was last seen alive at his home of Harkness Hall at around 9pm on June 3 by his grandson Ben, who had taken him a Sunday roast.
In the following days Cannon sold a Suzuki 4x4 for cash before the vehicle ended up in Bulgaria, while Taylor got rid of her mobile phone, the trial heard.
But WhatsApp messages found on Cannon's phone beginning in February last year showed the pair had enjoyed discussing sexually graphic and violent ways of seriously harming Mr
Taylor, who suffered from both tinnitus and arthritis.
They included fantasies of showering together as his blood ran down the drain and cutting his ligaments and fingers off.
On the night the landowner vanished, Cannon told Taylor he was "just watching Kill Bill 2 lol", with her replying "1 would be nice" with several smiling emojis.
In the months beforehand, he told Taylor he would "kill for" her and was afraid of losing her.
He went on to say: "You know what I'd like to do ... make love to you on his kitchen table with your pussy soaked in blood with him tied to a chair so he had to watch !!!!!!!"
"Then send him to hell."
Jurors rejected the couple's defence that the exchanges were pure sexual fantasy and had no basis in reality.
Prosecutor John Price QC said Cannon killed the cattle farmer himself while Taylor was probably at home, but she was equally guilty by encouraging him in the act.
The pair were also convicted of arson for partially torching Mr Taylor's Land Rover several days before his disappearance.
Cannon's work colleague, Gwyn Griffiths, 60, of Folkestone, Kent, who the court heard had so-called "people-pleasing" psychological traits, was cleared of conspiracy to murder after being
accused of discussing hiring a hitman with Cannon.
Taylor and Cannon, both of Hitchin, Hertfordshire, will be sentenced on Friday by judge Michael Kay QC.
Near Mr Taylor's remains, which were discovered still wearing Wellington boots and blue overalls, were found a ceramic teacup from his house, a bottle of Baileys liqueur, the remnants of a corn on the cob, and some rope cord, the court heard.
Michael Magarian QC, defending Cannon, said the items suggested Mr Taylor may have "gone to that spot like a picnic, almost" and argued death by suicide, accident, natural causes, or at the hands of someone else, could not be ruled out.
Jurors rejected his suggestion that the "the rather banal answer" was that Mr Taylor had got stuck in the wet and boggy ground by himself.
Judge Michael Kay QC had asked jurors to consider: "Has there been an amateurish attempt to make the scene appear as if there has been a suicide or accident?"
How the farmer died remains unexplained.
Forensic pathologist Dr Charlotte Randall found there was no sign of blunt force injuries, no gunshot or stab wounds, and no evidence of toxic substances ingested.
But there was a "possible fracture" to the hyoid bone, a delicate neck structure, raising the possibility of compression of the neck prior to death, the prosecutor told the court.
Taylor married Mr Taylor in 1997 and the couple had three children together.
She lived nearby, having acquired two of Mr Taylor's farms debt-free, Dog Kennel Farm and Mill Farm, along with some 220 acres, as part of a post-nuptial financial settlement after first filing for divorce in 2014.