Obviously, w/o seeing the situation, it will be hard to exacly diagnose the cause. The situation may be a relatively new one or may be one that's been happening over some time (as you say). BUT, to be sure I can cover a few more 'bases', I'll assume nothing.
Check that you have positive slope at the outer wall. The MINIMUM grade is 1/4" per foot for a distance of 3'. If water is manages to find its way back towards the house, this needs to be corrected. (And look for separation between the foundation and the ground. Water can 'run' into the separation, out of sight.) If you're over a crawlspace, check out that corner underneath. If you have access to the attic, check out the corner above the wall in question. Water can run DOWN the wall and pool at the base of the wall. Look at the ceiling above the room too and look for signs of dryrot. This may point you in the direction of the leaking roof.
Other than that, not much else to offer. My best to ya and hope this helps.
PS: If the roof is aged, it may be time for a new one.