I will be in Paris on a Sunday morning. If I attend the 8:30 Mass, can I stay to tour the Cathedral? How early should I arrive to gain quick access?
The cathedral of Notre Dame opens at 8AM every day. Since admission is free, you can stay in the cathedral as long as you wish.
....and be prepared for tourists, with cameras clicking, to wander thoughtlessly into the area up front where the Mass celebrants are seated.
There is usually a door marked "Messe" for those who want to attend Mass. I'd go about 20 min early because agents are checking everyone's bags.