As a seasoned veteran, he is an outstanding member of the force, but has no patience for incompetent people, which to him, is most other members of the team.
As he is the senior agent, he takes his orders directly from Jim Miller and passes them along to his team. However, as Adam Jensen is on his team and is the only augmented agent, Miller will bypass the chain of command and directly order Jensen himself, which does not sit well with MacReady.
To most people, MacReady usually comes off as rude, impatient and, if they are augmented, bigoted, though he is also fiercely loyal to his superiors, righteous and places his duty over his bigotry. Although it is difficult to earn the respect of MacReady, he is an extremely valuable ally to anyone that does.
He has a distrust of augmented people, and thus is wary of Jensen when he joins his TF29 team. If pressed by Adam, he reveals that part of the reason he distrusts Augs is because he lost 3 of his team members when the Aug Incident happened.