The character of Syd Barrett, who is played by Rachel Keller, is named after Syd Barrett of the rock band Pink Floyd, whose music was an important influence on the series for Noah Hawley. Barrett left Pink Floyd in 1968 due to mental illness issues.

Noah Hawley had hinted at the possibility of Charles Xavier appearing in the series ever since season one. Patrick Stewart had said that he would "absolutely 100%" reprise his role on the show if given the chance despite previously claiming that Logan (2017) would be his last time portraying the character. However, Hawley decided to focus on a much younger Xavier and Harry Lloyd was cast instead. Hawley had also strongly considered having James McAvoy reprise his role from the films as well, but it was decided that casting him would be "too expensive".

On February 4, 2019, it was announced that the show's third season will be its last, showrunner Noah Hawley always planned for the series to run for only three seasons and the decision to end it now comes as a result of him and the network wanting to close the story the way he always intended.

In one of the episodes as Cary Loudermilk (Bill Irwin) meets the man in the diving suit, he greets him in several languages. This is a reference to the book "Twenty Thousand Leagues Under the Sea" by Jules Verne, where Professor Pierre Aronnax also greets Captain Nemo in several languages not knowing what language he can speak.

The show is part of the X-Men cinematic universe as it connects to its future films while acting as a standalone series. However, it's hinted that it exists in its own universe and not part of the X-Men universe.

The second FX series from Noah Hawley. The first series is Fargo (2014).

FX gave the show an additional episode in season 2, making what was initially a 10 episode season an 11 episode season. The reason behind this was that at some point in the season so many scenes were shot that ended up on the cutting room floor so as to not have 60+ minute episodes. Noah Hawley asked FX to let him have an extra episode to use a lot of the material that he deemed necessary for the story.

The color red (in varying shades) figures predominantly in virtually every scene, either on a character's clothing or the set. This is reminiscent of a similar theme in The Sixth Sense (1999).

The character Syd Barrett is frequently shown early in the pilot episode with light reflecting around her head. This is a possible homage to the Pink Floyd tracks "Shine on you crazy diamond" parts 1 and 2 which were about former lead singer Syd Barrett who was also considered mentally ill.

The character Legion first appeared in the marvel comic book New Mutants #25

Lauren Tsai ("Switch") is also an illustrator, she has worked for Marvel Comics.

At the beginning of the series, David is diagnosed with schizophrenia. The other main character, Syd, is named after Syd Barrett (of the band Pink Floyd), had to leave his band after he was diagnosed with schizophrenia.

Jean Smart and Rachel Keller both starred in the second season of FX's Fargo (2014). Jean played Floyd Gerhardt who was the grandmother of Rachel's character Simone Gerhardt. Noah Hawley was so happy with their performances that wrote the roles for them.

Season three marks the acting debut of reality TV, model, and illustrator Lauren Tsai.

Jon Hamm, who serves as the second season's narrator, was originally cast as Mister Sinister in the upcoming X-Men film "The New Mutants", before his character was cut from the film. Whether the casting was meant to be a deliberate tie-in to the film, or just a pure coincidence has yet to be confirmed.

Kirby Morrow (Benny) previously played the mutant Cyclops/Scott Summers on X-Men: Evolution (2000).

In an interview with Collider, producer Simon Kinberg compared this series to Breaking Bad (2008).

Dr. Bird's car in season 1 is a right-hand drive Nissan President, manufactured some time between 1995 and 2002. A popular executive sedan in Japan, a left-hand drive version was sold in North America as the Q45 model of Nissan's Infiniti luxury division.

This is the 2nd Marvel project Hamish Linklater has been involved in, the first being Fantastic Four (2005) where he played Leonard, Victor Von Doom's assistant.

Dan Stevens also appeared in Beauty and the Beast (2017), which also featured Ian McKellen, who played Magneto in the X-Men films.

Originally, Noah Hawley could not direct the finale because he was too busy with the post-production of his feature film Lucy in the sky, but when the director hired to do so, Daniel Sackheim, had to back out, the cast of the show convinced him to direct it. Because he was still very busy, he ended up directing only some scenes, while the rest of the episode was helmed by John Cameron.

The title of the show is based on David Haller's codename in the X-Men Comics. David acquired the codename "Legion" due to having hundreds of personalities, each with his/her own mutant power; essentially making him a one man army.

In Marvel Comics, Legion (David) causes the alternate universe of a world without Charles Xavier. In this world or the "Age of Apocalypse" Xavier was killed and his legacy of starting the X-Men (et al) was changed. Magneto was the Headmaster of Xavier's School of the Gifted, Apocalypse rules the world, some good guys were evil (and vice versa), only one of the "X-Men" knew the alternate timeline was different/changed.

Aubrey Plaza's character Lenny Busker was originally written as a middle-aged, drug-addicted man in a mental hospital.

Legion (David Charles Haller) is a character from Marvel Comics. He is the mutant son of Professor Charles Xavier and Gabrielle Haller.

In the montage of David growing up, David sets an experiment on fire while at school and later, a convenience store burns behind David as he walks away. In the comics, pyrokenesis is a power that David commonly uses.

In "Episode 7" David describes his fight between his father and "The Monster". His father is drawn as a bald-headed mutant in a business suit and David then makes fun of his father's British accent. This is an allusion to David's father in the comics (who is Charles Xavier) and is played Patrick Stewart with a British accent in the X-Men movies.

First live-action appearance of the powerful psychic mutant entity known as "The Shadow King" aka Amahl Farouk.

Easter Egg: throughout the series a number of X's can be seen in the background of scenes.

During the episode where they first see Lenny in David's memory, the wall Lenny is sitting in front of has the face of Benny.

David Haller is the son of Charles Xavier in the comics. The character has a severe mental illness in the form of a dissociative identity disorder, which grants him multiple personalities. James McAvoy who plays Xavier in X-Men: First Class (2011) also stars in Split (2016) and Glass (2019) as a man who suffers from a very similar condition.

In one episode, in a discussion regarding a major location in the series, Aubrey Plaza's character says "I mean, what kind of a name is Summerland?". Oddly enough, there is a town called Summerland in British Columbia, roughly 250 miles north of where the series is filmed.