Somebody found my blog on Sunday by searching for the phrase, "missing part of Sunday Mass." The search engine brought up this old post of mine (along with some others about missing Mass generally on various sites).
I know what it is like to be scrupulous about such things; in fact, I told the mother of a young child just the other day that my own mother used to say, jokingly, that she felt like an atheist until each child was about two, because of all the time she spent in the vestibule with the baby and not really able to participate fully at Mass. Of course, those caring for small children have a good reason why they may not be able to attend Mass at all, so I think "vestibule duty" is setting a terrific example of heroic suffering and participating as much as one is able, anyway.
I can't, of course, assume that the person asking the question is actually being scrupulous, though. In fact, I can't assume anything at all about his or her motives in writing this query; but since the query existed, I figure it can't hurt to put out an answer of sorts--as always, subject to correction by the Church if I misstate anything.
So: is it a sin to miss part of a Sunday Mass?
The old answer used to seem to be that so long as you came in either by the Gospel (in some versions) or by the Offertory (in others) you were fine. As Jimmy Akin points out in this post of his from about four years ago, though, the liturgical law does not at present draw a clear line at which point a person has missed so much of a Mass that he must go again in order to fulfill the obligation to attend Mass on Sunday--and, of course, all Catholics are obliged to attend Mass on Sundays and Holy Days of Obligation unless prohibited for a serious reason (and the Catechism lists such things as illness and the care of children as examples of some serious reasons one may legitimately miss Mass on a Sunday). So, today, the questions about whether one has satisfied the obligation to assist at Mass if one has arrived late will have varying answers depending on many circumstances, some of which I'll discuss here.
The first question is: Have I arrived late to Mass or missed some part of the Mass through my fault and for no good reason? A late arrival, for instance, because one is attending Mass at an unknown church in an unfamiliar town and one gets lost in traffic on the way is probably not one's fault; a late arrival to the last Mass of the day because one was drinking heavily the night before and slept through one's alarm clock might be. Or, for another example, missing part of Mass because one's toddler suddenly needs an urgent diaper change or because one is beset by a coughing fit that is distracting to others (and that sends one in search of the water fountain in the parish hall) is clearly not a fault, but missing part of Mass because one is bored and decides to go surf the Internet on one's phone out in the parking lot clearly is. And then there is the question of early leaving: if one has to leave Mass before the end of Mass because, for instance, one is a nurse and must be at work by 10:00 a.m. and the visiting missionary priest gave a thirty minute homily at the 8:00 a.m. Mass instead of the usual five minute one, one is fine; so is someone taken ill during Mass, someone who must leave with a screaming child, and so forth. But the person who leaves after receiving Communion just to be first in line at a popular brunch spot probably needs a bit of soul-searching.
The second question is: if I have missed a significant portion of the Mass, must I attend another? Here, I can share an experience: long ago, we went for the first time to a church an hour away from home with three children under age 3 on Palm Sunday. We got lost on the way there, and arrived either as the Passion reading was concluding or the homily was beginning (at this space of time, I can't recall exactly). The church was packed, and we stood in the back with the girls. The Mass was a noon Mass (we had thought that would give us plenty of time to arrive in the distant city and find the church, but we were wrong). There was no 5 p.m. Mass closer than another city three hours away from the city we were in, and no Mass later than that noon one that was even remotely possible for us to attend, even if three young toddlers could have made it through a late afternoon or evening Mass--there simply wasn't another Mass available. So although it wasn't ideal for us to have missed so much of the Mass, we realized that this was the only Mass we were going to be able to attend that day, and we made the best of it.
Sometimes, of course, arriving late or missing a significant part of Mass doesn't mean you can't go later to another Mass, and each person has to decide what is prudent and what is necessary. I have known young moms who sneak off to an evening Mass just because the morning Mass complete with young children was such a distracted and interrupted experience that they crave another encounter with our Eucharistic Lord, and while it may not be strictly necessary, it's never a bad thing, provided it can reasonably be done.
The bottom line, to me, is this: if you did not intend to miss part of the Sunday Mass, and you ended up missing a part for no fault of your own, you are fine. If you miss a significant part (even through no fault of your own) and can attend another Mass you are always free to do so, but whether you are obligated to do so will depend on individual circumstances to such a degree that you may have to consult with your pastor to be sure. Certainly if you miss part of Mass for a serious reason, such as needing to rush out of Mass with a child displaying signs of illness, and the child remains ill for the rest of the Sunday, you have the ordinary sort of serious reason which removes the obligation to attend Mass. If the reason is less serious and you can easily attend another Mass, certainly you may choose to do so. But if you missed a significant part of the Mass through laziness, indifference, or something else that is definitely your fault, the situation is somewhat different; if the fault is habitual, you may find it prudent to address it in the sacrament of Penance.