Roger Cooper, 41, allegedly decided to take Sameena Imam "out of his life" after she issued him with an ultimatum to leave his long-term partner.
It is alleged Mr Cooper, of Coventry, believed he would be sacked if Ms Imam, from Cardiff, exposed their affair.
He is standing trial at Birmingham Crown Court with David Cooper, 38, of Leicester. Both men deny murder.
Jurors were told Roger Cooper, manager of Costco's Coventry warehouse, may claim to have travelled to Leicester to pick up a Star Wars figure as a present from his brother.
Opening the case against the two men, prosecutor Timothy Spencer QC said: "He did pick (the Star Wars figure) up but we say the reality of that journey is that it had a much more sinister motive.
"It was really about what was going to happen to Sameena Imam."
CCTV footage played to the jury showed Ms Imam, the store's marketing chief, and Roger Cooper leaving Costco's Coventry branch within minutes of each other at about 16:00 GMT on 24 December last year.
The Crown claims Ms Imam then left her BMW in the local area and was driven to Leicester before being killed within hours.
Mr Spencer said of the defendants: "They thought they had got away with the killing, they thought they had covered their tracks and traces. They were wrong."
At the time of Ms Imam's death, the court heard, Roger Cooper was also having relationships with two other women, his live-in partner Susan Potts and Costco colleague Sinead Sweeney.
Ms Imam, who lived in south Wales and worked at Costco outlets in Coventry, Southampton, Bristol and Cardiff, was reported missing after failing to attend a family gathering in Essex on Boxing Day.
During his opening address at the start of a two-month trial, Mr Spencer added: "From Boxing Day onwards a missing person inquiry was triggered."
A cash-and-carry boss killed one of his three lovers on Christmas Eve with the help of his brother, a court has heard.