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 From the Catholic Spirit's Catholic Hotdish blog

Is Confession valid if we don’t do the penance?

Have you ever had to leave right after confession, intending to do your penance as soon as you could—but then you forgot? Or has a priest told you to do a charitable or self-sacrificial act for your penance instead of saying a prayer and because you couldn’t do it at that moment it slipped your mind? In both cases is the absolution valid?
The answer is, it depends.
As with many of the laws and norms governing the Christian life, your level of culpability depends on where your heart is. According to canon law, “the confessor is to impose salutary and suitable penances in accord with the quality and number of sins, taking into account the condition of the penitent. The penitent is obliged to fulfill these personally.” (Canon 981)

What makes a confession invalid

We’re obliged to do the penance, but what  if we accidentally don’t?  The conditions below make a confession invalid, according to a book co-authored by Cardinal Donald Wuerl:

• No true sorrow for sins and lack of intention to avoid grave sin in the future,
• Deliberately neglecting to confess all grave sins, or
• Refusing to do an assigned penance.

So it seems that forgetting to do a penance doesn’t carry the same weight as willfully refusing to do it, and therefore doesn’t invalidate the absolution. But Father John Hardon points out that through centuries of Church teaching, the following have been required of those who receive the Sacrament of Reconciliation:

• They must be truly sorry for their sins, at least out of fear of God’s punishments;
• They must confess their grave sins, or (if there are no mortal sins) at least some venial sin(s) from their past life; and
• They must perform the penance which the confessor gives them.

Importance of the penance

Receiving absolution isn’t the whole story, however. When it comes to making amends for our sins, the penance given in confession plays an important role.
The Catechism states: “Absolution takes away sin, but it does not remedy all the disorders sin has caused. Raised up from sin, the sinner must still recover his full spiritual health by doing something more to make amends for the sin: he must ‘make satisfaction for’ or ‘expiate’ his sins. This satisfaction is also called ‘penance.’” (CCC 1459)
I’m not quite at the point where I’m tying strings around my fingers to remember things like Uncle Billy in It’s a Wonderful Life but I have forgotten to do a penance – or worse, done it half-heartedly. I guess in those cases it might be good to think about why we’re going to confession and who we’re apologizing to.