Leaving Mass Early

The answer varies based on the reasons for leaving Mass early.

If someone leaves Mass early because of an emergency or to care for someone who is sick then there is no sin involved.

If someone leaves Mass early simply to get out of the parking lot sooner then it is most likely a venial sin.

If someone leaves Mass early because of a lack of respect for the Eucharist, the Lord, or fellow parishioners it could indeed be a mortal sin.