Dan Stevens stars as Haller, with Rachel Keller, Aubrey Plaza, Bill Irwin, Jeremie Harris, Amber Midthunder, Katie Aselton, and Jean Smart also starring in the season. FX ordered a pilot for Legion in October 2015, which Hawley wrote and directed. The show was picked up to series in May 2016. Filming took place in Vancouver, with a focus on in-camera effects over visual effects. Hawley wanted to show Haller as an "unreliable narrator", with the series mixing retro and modern designs, using unconventional filming and musical techniques, and being structured so the audience is unsure what is real. The narrative becomes more clear throughout the season as Haller gains knowledge, and the villainous Shadow King is revealed. The latter takes several forms, including one portrayed by Plaza.
The season premiered at the Pacific Design Center on January 26, 2017, ahead of its FX debut on February 8. It ran for eight episodes, until March 29. The season received critical acclaim for its cast, particularly Stevens; Hawley's visuals and design; and the nonlinear, unreliable nature of the storytelling. Several critics did note that the latter aspects would not be for all viewers, and some criticized this as just the latest in a trend of series with unreliable narratives. Legion was renewed for a second season on March 15, 2017.
In October 2015, FX ordered a pilot for Legion, with Marvel Television and FX Productions producing; FX Productions would handle the physical production. Hawley was set to write the pilot, and executive produce the series alongside X-Men film producers Lauren Shuler Donner, Bryan Singer, and Kinberg, Marvel Television executives Jeph Loeb and Jim Chory, and Hawley's Fargo collaborator John Cameron. Steve Blackman, Alan Fine, Stan Lee, Joe Quesada, and Karim Zreik also executive produce. By January 2016, FX President John Landgraf was confident that the series would be picked up by the network, probably for ten episodes; that May, FX ordered an eight-episode first season of Legion. Landgraf later explained that only eight episodes were ordered because FX wanted Hawley to run the series at his own pace rather than try and "pad" it out.
Hawley first developed the idea of a series about Haller with Kinberg, after asking the question "is there an interesting show in this genre, and is there a character in that show?" He noted that series such as Marvel's Netflix shows and Preacher had not been released at that point. Hawley was specifically interested in exploring the character of David Haller because of his mental illness, and for the potential of the series to depict his unique mindset. He pitched the series as "a deconstruction of a villain ... and a love story".
Hawley's first thought when "looking at the genre is if we remove the genre, is there a compelling show you want to watch there?" He decided to center the series on the "idea of this epic love story and then putting the genre back into it" and layering elements such as mental illness and special abilities on that foundation Hawley's initial script for Legion was described by Donner as "less fractured", "cohesive much more regular." However, he quickly reconceived the series "and decided more Eternal Sunshine of the Spotless Mind, Terrence Malick, more whimsy, more impressionistic and went in that direction." This was in part due to Hawley's desire to depict a "more existential exploration" and a "surreal or dreamlike quality where it's not just about running and kicking." He was inspired by the works of David Lynch to have the series' structure reflect the content of the story. Explaining, Hawley noted that the series' protagonist "doesn't know what's real and what’s not real, the audience should have the same experience". Hawley added that "there is this Alice in Wonderland quality to it, of a story within a story." To acclimatize audiences to the different style of the series, taking into consideration that the show was given a late airtime and so "it’s on at the end of their very long day", Hawley had several of the early episodes begin with a "hypnotic quality ... often a little ethereal voice-over, to give the sense that we are not rooted in a physical time and place." The intent was to put audiences "in the state of mind to watch Legion."