Mockford, 60, who promoted new types of green fuels for the company, was shot dead in front of his wife on Sunday 14 October, according to The Daily Telegraph.
He was shot three times as he and his wife left a restaurant in Neder-over-Heembeek, a suburb in the city. His wife was beaten but left alive.
One family member told The Daily Telegraph they believed Mockford had been killed in a professional hit.
Belgian police have imposed a news blackout and the Belgian prosecutor's office was unable to give any explanation or details about the killing, or its investigation
Mockford was born in Leicestershire and had worked for ExxonMobile since the 1970s. He is survived by his wife and three children from his previous marriage.
A spokesman for ExxonMobil said: "We were shocked by the tragic death of Nick Mockford, one of our employees, a fortnight ago on Sunday 14 October, in Brussels.
"Mr Mockford, a British national, was a department manager at our office close to Brussels, but we have no indication that the incident was work related.
"Our thoughts are with his family, friends and colleagues and we are supporting them as best we can at this very difficult time."