What is the tone or mood of this short story?

Fitzgerald's story in many ways prefigures The Great Gatsby because Dexter possesses many of the same yearnings that Jay Gatsby does. Both men are outsiders who find a way into the worlds they aspire to, only to become disillusioned by what they find.
The mood of the story is not fixed; it could be said that the story begins with yearning and optimism as readers witness Dexter's drive to surpass the successes of the people he serves at the country club. His ambition will not allow him to simply join them; he must best them. However, as the story unfolds and Dexter becomes more financially successful and gains access to the upper reaches of society, including the milieu of Judy Jones, his restlessness does not resolve. Dexter is unable to achieve self-actualization in any enduring sense. Dexter's last words are a haunting expression that suggest the meaning of life is found in the journey of becoming, not in the acquisition of what one thinks he wants:

"Long ago," he said, "long ago, there was something in me, but now that thing is gone. Now that thing is gone, that thing is gone. I cannot cry. I cannot care. That thing will come back no more."

The tone of the story as it concludes is despairing and melancholic.