And at the same time they knocked another nail in Grimsby's hopes of staying in the Football League as they made it nine games without victory and a fourth consecutive defeat for the Mariners.
Wet and windy conditions in north-east Lincolnshire made good football virtually impossible and it was difficult to judge Brentford's title credentials on this form.
But what was clear is that manager Mike Newell faces a mammoth task if he is to prevent Grimsby from dropping out of the league for the first time since they entered it back in the 19th century.
The only goal came when Ryan Dickson put a good ball in from the left and although midfielder Adam Newton should have finished it off, his touch meant that the ball fell to MacDonald and it wasn't the sort of chance he misses as he snapped up his 18th goal of the season.
Grimsby's only attempt on goal in the match apart from a Jamie Clarke free-kick which was easily saved was a long clearance from on-loan keeper Wayne Henderson which went first bounce over the Brentford bar.
If the first half was dire the second period got even worse and not even a triple substitution by Grimsby midway through could add any life to the proceedings as the Bees saw out the 90 minutes with not the slightest difficulty.