Costco store manager Roger Cooper, 41, and ex-soldier David Cooper, 39, were convicted of killing Sameena Imam, 34, at Birmingham Crown Court.
Her body was found buried at an allotment in Leicester in January after she was killed with chloroform.
Ms Imam had been having a two-year affair with Roger Cooper and issued him with an ultimatum to leave his partner.
An eight-week trial heard that Roger Cooper spent at least a month plotting to kill Ms Imam - one of his three lovers - to prevent her exposing their affair.
The men bought poisonous metals, identified a shallow grave and communicated in text messages written in a Star Wars-based code before killing Ms Imam.
Roger and David Cooper remained calm and did not look at each other as they were unanimously convicted by a jury on Tuesday.
Ms Imam is believed to have been overpowered on a sofa at David Cooper's home in Leicester after being driven to the property from Coventry by his brother.
She was reported missing by family members in Essex after failing to return home for Christmas, sparking a major police inquiry.
Detective Chief Inspector Caroline Marsh said: "I would say this is the most callous offence I have ever investigated.
"Most murders that occur are spontaneous events driven by anger or jealousy, whereas in this case what we uncovered was weeks and weeks of planning to kill Sameena, numerous attempts on her life and a really unusual cause of death.
"It's been a particularly challenging case and a particularly complex investigation and we are extremely grateful to the jury, who have listened to the case and given us the verdict we've had today."
Two brothers accused of murdering a cash-and-carry manager from Cardiff on Christmas Eve have been found guilty.