Nicholas Mockford, who had reportedly worked for the US supermajor for almost 40 years, was with his wife when he was gunned down outside a restaurant in the Belgian capital on 14 October.
Belgian police did not reveal many details about the case which has largely led to a delay in it being reported.
Mockford, who is understood to have been a marketing manager in ExxonMobil’s green fuels division, and his wife left the Da Marcello Italian restaurant in a Brussels suburb on the evening of 14 October.
As they crossed the street to go to their Lexus vehicle, they were reportedly approached by two men who first attacked Mrs Mockford before firing either three or four shots at her husband.
Witnesses said they saw the assailants leave the scene carrying at least one motorcycle helmet.
Mrs Mockford was left beaten and bloodied by the incident in which the assailants tried to steal her handbag.
Police have remained tight-lipped about the case but are reportedly considering multiple motives.
A statement from ExxonMobil read: “We are shocked by the tragic death of one of our employees on Sunday, 14 October in Brussels.
“Our thoughts are with his family, friends and colleagues and we are supporting them as best we can at this very difficult time.”
UK newspaper The Telegraph quoted an unidentified relations of Mr Mockford as saying: “We are all confused about what has happened. Nick was a genuinely lovely, clean-cut, mild mannered, family man.
“I don’t think he would put up a fight or argue with someone trying to steal his company car.
“He was shot so calmly and so quickly, it smacks horribly of a professional hit, but we can’t fathom why. He isn’t the type to cave in to blackmail and it just doesn’t compute.
“He was always very tight-lipped about what he did, even when sitting around with the family.”
The UK Foreign & Commonwealth Office said: “We can confirm the death of a British national in Brussels on 14 October and are providing consular assistance,” saying it is an investigation for the Belgian police.