The ending of the story is a traditional O. Henry-style twist ending, which confounds the reader's expectations through the use of situational irony. In particular, the situational irony of Johnsy's survival and Behrman's death prompts readers to rethink their assumptions about the characters, especially Behrman, and to consider what makes for a truly meaningful legacy.
When Johnsy sees that the last leaf remains on the vine after consecutive nights of violent wind, she concludes,
“Something has made that last leaf stay there to show me how wicked I was.”
In particular, Johnsy believes that she was "wicked" to want to die, and soon after this, her attitude—and her health—take a turn for the better, with the doctor finally giving her a hopeful prognosis. At this point, readers don't know any better than Johnsy does how the leaf inexplicably stayed in place.
But, the next day, Johnsy learns that Behrman has died after spending a night out in the storm, painting the leaf on the wall to disrupt Johnsy's fatalism (connected to the leaf's symbolism) and inspire her to live. Sue tells Johnsy the news this way:
“I have something to tell you, white mouse," she said. "Mr. Behrman died of pneumonia today in the hospital [...] His shoes and his clothes were wet through with cold. They couldn’t imagine where he had been on such a dreadful night.[...] [L]ook out the window, dear, at the last ivy leaf on the wall. Didn’t you wonder it never fluttered or moved when the wind blew? Ah darling, it's Behrman’s masterpiece—he painted it there the night that the last leaf fell.”
This ending is filled with irony. Throughout the story, readers have been primed to expect Johnsy's death because of her fatalism and the leaf's ominous symbolism. Instead, it is Behrman (who earlier scorned her obsession with the leaf) who dies. It's also ironic that the literal "last leaf" did fall, but that it's a mere depiction of a leaf that helps raise Johnsy's spirits and encourages her to keep living. These ironies suggest that Johnsy's health, and human survival in general, is deeply connected to a sense of hope, and that loving, self-sacrificial friendship—more than ambition—is what makes life most worth living.
It's also important to note that Behrman had earlier spoken of his desire to create his "masterpiece," by which he intended to make a name for himself among art's greatest masters. As it turns out, however, his "masterpiece" is a simple painting of a little leaf, intended only for Johnsy's eyes—and intended not to establish his own legacy, but to give the young woman a reason to live. This layer of situational irony undercuts readers' earlier expectations of Behrman as a cantankerous, skeptical old man mainly concerned about himself; rather than continuing to grasp at a legacy, he was willing to put his own life on the line and die a painful death in the hope that his young friend would take heart and survive.
