I was baptized and confirmed 7 years ago. Before my conversion, I committed a mortal sin which I have never confessed to a priest. Do I need to mention this sin at confession?
Through baptism all of your prior sins were forgiven, so it is not necessary to confess those sins. The Catechism of the Catholic Church explains:
By baptism all sins are forgiven, original sin and all personal sins, as well as all punishment for sin. In those who have been reborn nothing remains that would impede their entry into the Kingdom of God, neither Adam’s sin, nor personal sin, nor the consequences of sin, the gravest of which is separation from God. (CCC 1263)
Confession is for sins committed after baptism:
Christ instituted the sacrament of penance…above all for those who, since baptism, have fallen into grave sin, and have thus lost their baptismal grace and wounded ecclesial communion. It is to them that the sacrament of penance offers a new possibility to convert and to recover the grace of justification. (CCC 1446, emphasis added)