There is more ambiguity in the second version than the first, though from context it ought to be easy to know which meaning was intended.
I didn't know that she was going to be there until 5 minutes before service, and then have to leave.
I didn't know that she was going to be there until 5 minutes before service, when she surprised me by walking into the kitchen.
The first seems clearer, that you had no idea she would turn up until she did, just before some timed event.
btw, 'before service' implies a restaurant/kitchen. If you meant a church, then you would use 'before the service'.