The story is Aesop’s; the sentence is not. “No act of kindness, no matter how small, is ever wasted” is a later moral attached to the fable of the Lion and the Mouse. It is not in the Greek text (Chambry’s edition), nor in the English translations by George Fyler Townsend (1867), Joseph Jacobs (1894) or V. S. Vernon Jones (1912). The shorter saying “A kindness is never wasted” is older than its use with this fable; the earliest edition we found that prints it as the fable’s moral is The Aesop for Children (1919).
What it means
Small kindnesses matter because you cannot know which ones will come back, or how. In the fable a lion lets go a mouse that seems far too small ever to repay him; later, when hunters tie the lion up, the mouse gnaws through the ropes and sets him free.
Where it comes from
In the Greek collection (Émile Chambry’s edition, where the fable is titled “the lion and the mouse who returned the favour”), the mouse ends by telling the lion that “there is gratitude even among mice,” and the moral reads: the story shows that, when fortunes change, the very powerful come to need the weaker. Its lesson is that the strong may one day depend on the weak, not that kindness is never wasted.
The standard English versions keep that emphasis. Townsend (1867) ends with the mouse saying “now you know that it is possible for even a Mouse to confer benefits on a Lion”; Jacobs (1894) adds the moral “Little friends may prove great friends”; Vernon Jones (1912) ends “even a Mouse can help a Lion.” The children’s book The Aesop for Children (Rand McNally, 1919), illustrated by Milo Winter, prints “A kindness is never wasted” under this fable and under The Ant and the Dove. That was already an English saying: in E. Nesbit’s novel Harding’s Luck (1909) a character says, “A kindness is never wasted, so they say.” The full sentence, with “no matter how small”, appears by 1947 as the “Application” of the fable in an edition illustrated by Fritz Kredel (Grosset & Dunlap), according to full-text search; we have not seen the page itself.