The mood of the poem seems to shift with the speaker's mood. At first, the poem's mood seems to match his (or hers), but then it breaks from the speaker's mood pretty dramatically. First, he has "a heart fit to break"—fearing, it seems, that Porphyria will not come because of the bad weather—and so he describes the wind as "sullen" and "spite[ful]" in its attempts to "vex the lake." His mood is low, and so his morose and irritated descriptions of the weather (the weather that could keep his lover away) establish the sullen mood for the first five lines.

When Porphyria arrives, in line 6, she lights a fire, making the "cheerless grate / Blaze up," so that "all the cottage [becomes] warm." The mood begins to lift with her entrance and as she works to please and pet the speaker (who playfully does not respond to her initial call). She tells the speaker that she loves him, but

. . . she
Too weak, for all her heart's endeavour,
To set its struggling passion free
From pride, and vainer ties dissever,
And give herself to me forever.

Porphyria says that she will not be able to stay with the speaker, her lover, due to some vanity on her part (perhaps the narrator is of lower social status and their relationship would cost her social position). We might await a change in mood here because she has just told him, essentially, that their relationship must come to an end. Instead, however, of the speaker becoming depressed, he seems to become happier. The narrator describes "to-night's gay feast" and how he feels "Happy and proud" to know that "Porphyria worshipped [him]." His personal mood elevates as he decides to strangle Porphyria with her hair and, in this way, keep her with him forever. Meanwhile, the mood of the poem becomes completely chilling as we realize that he has murdered her in order to compel her to stay with him. The mood of the poem becomes threatening and unpredictable, as we have no idea what else this speaker is capable of.

Porphyria's Lover is typical of Robert Browning's dramatic monologues in that the poems reveal as much about the dramatic narrators as about the putative subjects of the poem. Several other poems by Browning, of which the best known is probably "My Last Duchess", start with what appears to be a narrator describing his love for a woman. The opening of "Porphyria's Lover" is foreboding, with the weather mirroring the narrator's mood of despair. When Porphyria first appears, the mood changes to tender and romantic:

When glided in Porphyria; straight
She shut the cold out and the storm,
And kneeled and made the cheerless grate
Blaze up, and all the cottage warm;

but the lover's jealousy eventually prevails, and the violence foreshadowed by the opening surfaces:

Made my heart swell, and still it grew
While I debated what to do.
That moment she was mine, mine, fair,
Perfectly pure and good: I found
A thing to do, and all her hair
In one long yellow string l wound
Three times her little throat around,
And strangled her.

The mood of peace at the end, with the narrator sitting with the corpse of Porphyria and the voice of God (and perhaps weather) quiet, adds an eerie twist to the poem.