Ba broke the deadlock in the 33rd minute when Mike Williamson headed Danny Simpson's free-kick across the face of goal to leave Ba with the simplest of chances.
Magpies boss Alan Pardew made a series of changes at half-time and French Ligue 2 side Monaco almost equalised shortly after the restart when Valere Germain headed against the post.
Ba almost extended Newcastle's lead in the 71st minute when he headed just wide from a Ryan Taylor corner.
And substitute Gabriel Obertan came close in the dying seconds when he burst clear but fired straight at Monaco keeper Daniel Subasic.
