Danny Graham opened the scoring for the visitors before two goals in 12 minutes from Scott McDonald and Andrew Taylor gave Boro a vital three points.
Mowbray said: "It is a good cushion but that doesn't mean anyone here will be relaxing.
"It is a big gap, we know that, and some of those teams behind us now need to win three games to close the gap on us and with every week they have fewer games to do that in.
"But we know what this game is like and what this league is like and we can't take anything for granted. We know there is a long way to go.
"We've got a break next week but then we have a big April with eight games and we know we have to keep our standards high and have to keep picking up points. But I thought we deserved it. We played well, we worked hard and we kept going. I'm pleased."
Source: PA
Source: PA