Yes, missing Mass is a mortal sin. Here is why. For almost 250 years, it was a crime punishable by death to go to Mass. Even the mere accusation of being a Christian and failing to denounce it might have brought martyrdom.
The basic test for determining whether a person was Christian was whether that person regularly shared in the Church’s worship. For the state, a person who professed Christian beliefs but did not express them in prayer posed no danger. For the Church, beliefs not expressed in regular Eucharistic worship were meaningless. The state regarded sharing in the Christian assembly as an act of treason, a capital crime. The Church regarded sharing in its worship as the supreme positive affirmation before God of the Christian life
No Time For Church
Today, many profess faith yet do not practice Eucharistic worship (go to Mass). They are too busy…
Scripture commands us to “work out your own salvation with fear and trembling, for God is at work in you, both to will and to work for his good pleasure” (Phil. 2:13f).
Scripture explains that “as often as you eat this bread and drink the cup, you proclaim the Lord’s death until he comes” (1 Cor. 11:26)
Hebrews explains, “And this will was for us to be holy by the offering of his body made once for all by Jesus Christ” (Heb. 10:10). The offering made once for all is continually repeated on our behalf by the celebration of the Eucharist, thus continuing the working-out of our redemption. “By virtue of that one single offering, he has achieved the eternal perfection of all whom he is sanctifying. ”
That is why it is a mortal sin to miss Mass. By choosing to stay away from Mass on Sundays and Holy Days of obligation, we deliberately turn our backs on Christ and the process of redemption. You are literally saying NO
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