(Very paraphrased from a conversation at least 20 years ago): A priest friend of mine who has his degree in Liturgy once said that to be considered as having attended Mass, you have to get there no later than the Gospel, otherwise you’ve missed the Liturgy of the Word and are not technically considered to have attended Mass.
You can still receive Holy Communion, but if this occurs on a Sunday, technically you would not have fulfilled the Sunday obligation.
I personally feel, though, that if the lateness is through no fault of your own, and there is not another Mass to attend, that the Lord understands and forgives. (I would mention it in Confession, but I would not feel guilty as though I had stayed home just because I hadn’t “felt like” going to Mass.)
Of course, the ideal is to get there awhile before Mass begins in order to quiet your spirit and prepare yourself.
However, it’s no longer as it was in the old days when every Sunday was more or less the same, or in the older days when everyone just walked over to the village church – there are so many variables in today’s world (travelling and having been given wrong Mass times, regular route suddenly closed with unannounced road work… or the Marathon !… or another parade – don’t start me going on about my neighborhood!, missing a flight or there being a delay, etc.), and so we need to be a little extra vigilant about getting to Mass.
When we’ve done our very best, and we still have either missed Mass or gotten there very late, the Lord knows our hearts, He knows when we didn’t want to be late or miss and when we were careless. He knows how hard we’ve tried (or not), and knows if there is any guilt on our part. Don’t beat yourself up for no reason when there is no reason, because Jesus knows our hearts.
*“I wish that my heart could beat, that I could live and breathe only for Jesus, I wish that my tongue could utter no other name than that of Jesus; that my eye could see only Jesus; that my pen could write only about Jesus, and that my thoughts could soar to nothing but Jesus. I have often wondered where on earth there might be something on which I could center my love. But neither on earth nor in heaven do I find any such thing but only my beloved Jesus… I am the fruit of your passion, Jesus, born of your wounds. O Jesus, seek me in love; I no longer possess anything; you have stolen my heart…” *- St. Gemma Galgani