Delusional Thomas was one of the many alter egos of Pittsburgh rapper Mac Miller, easily recognized by his distinct pitched-up vocals and his graphic horrorcore imagery portrayed through his lyrics. When asked about Delusional Thomas and his other aliases, Mac shared,

I like to create characters and flesh them out all the way, ‘cause everything that’s done as Mac Miller is factual and based off factual information. Delusional Thomas is a character I created who’s kind of, like, a psychopathic murderer.

Delusional Thomas first emerged as an unnamed high-pitched voice throughout Mac’s sophomore album Watching Movies with the Sound Off (2013). Mac gave insight into his original vision for the character in a track-by-track interview for the album when discussing its intro, “The Star Room”:

To me, I just wanted to start the album off really honestly, you know? And so, like, the two voices— there’s a voice, and that’s kind of like, like a subconscious or a voice in my head, right? And it’s like a different “character” that’s starting the album, you know? If where, like, at a circus-type event, there’s a person that comes out first and he’s really weird and creepy, and he’s like, “Is everyone ready?” And everyone’s kinda like, “What the fuck’s going on? Who the fuck is this lil' dude?” And then the curtain opens, and the show starts, you know, that’s kind of that guy.

On June 25, 2013, exactly one week after the album’s release, Mac announced he was producing a project for a rapper named Delusional Thomas. On Halloween night, Mac released the horrorcore mixtape Delusional Thomas, revealing Delusional Thomas to be the pitched-up voice from WMWTSO. However, compared to his lyrical content on the album, Delusional Thomas' self-titled mixtape contains extremely explicit and violent themes in line with the horrorcore genre.

After the development of his self-titled mixtape, Delusional Thomas' voice could be heard in nearly anything Mac touched inside his Sanctuary studio, including vocal appearances throughout Mac’s seminal mixtape Faces, a track on his posthumously-released album Balloonerism, and an official feature on Ab-Soul’s 2014 album These Days…

When Mac signed a deal with major label Warner Records in late 2014 and moved from Los Angeles to New York the following year, he began to steer away from officially using the Delusional Thomas moniker, and after he appeared in a handful of SoundCloud singles in 2015, Delusional Thomas was never officially credited by Mac again. However, many of Mac’s fans speculate whether Delusional Thomas' voice can be heard in later works of his.