2016–present: Westworld and career progression
Simpson starred on the first season of SundanceTV's dark comedy-drama Hap and Leonard in 2016. Den of Geek felt he displayed a "raw, manic energy" in his portrayal of Soldier—a psychotic drug dealer—that was "by turns infectious and terrifying", adding, "Seriously, [Simpson] is such a great bad guy". That same year, he appeared in a principal role on the debut season of HBO's science fiction drama series Westworld. His portrayal of William, a businessman who visits the titular Wild West-themed amusement park and falls in love with one of its android inhabitants, was described as "spellbinding" by Maureen Ryan of Variety. Simpson was once again nominated alongside his co-stars for a Screen Actors Guild Award in 2017, while his work on the show's second season earned him an Emmy Award nomination for Outstanding Guest Actor in a Drama Series. Speaking of the pressure he felt being part of the hit show, Simpson said, "I think [a lot of us fear] that we won't be able to sleep at night if we deliver work that we're not happy with … Westworld [is] a lot of responsibility. It's a huge show. There's so much money and publicity behind it … and [the creators] need us to show up and be as amazing as possible. So the fear is a factor, because you look over and you see Anthony fucking Hopkins … It leaves me wondering: 'How the hell did I get here?'".
In 2017, Simpson appeared as Walton, a lieutenant aboard the titular spaceship ("USS Callister") in the opening episode of the fourth season of British anthology sci-fi series Black Mirror. In their review, Den of Geek called Simpson "one of [television's] best-kept secrets" and remarked that his performance "pops off the screen". His portrayal of Walton earned him a nomination for the BAFTA Award for Best Supporting Actor the following year.
Simpson starred as the real-life Russell Poole in Unsolved, a ten-part miniseries based on the 1990s murders of rappers Tupac Shakur and Biggie Smalls, which ran on the USA Network between February and May 2018. IndieWire praised the "elevated artistry" of Simpson's portrayal of the LAPD detective, while Vulture said in their review:
In an ensemble this solid, it can be challenging for one performance to emerge as a standout. But Simpson's does because he so carefully calibrates Poole's intensity, dialing it up by slight degrees in each episode until he's radiating with panicky determination ... Simpson physically and emotionally illustrates [the character's] internal struggle beautifully.
Simpson's next roles were in David Robert Mitchell's neo-noir black comedy Under the Silver Lake (2018), the action thriller Unhinged (2020), and the comedic crime drama Breaking News in Yuba County (2021). He also headlined the ten-part Epix series Perpetual Grace, LTD in 2019, earning strong reviews for his portrayal of James Schaeler, an ex-firefighter embroiled in a conspiracy to scam a corrupt pastor; Darren Franich of Entertainment Weekly described him as "an endearingly slippery protagonist, looking terrified, sad, amused, and exhausted all at once".
Between April and July 2022, Simpson appeared as CIA agent Spencer Clay in Showtime's adaptation of The Man Who Fell to Earth, which ran for a single season. In a mixed review of the series, Vulture commented that it felt "unsure" of itself, but said of Simpson, "[he tears] into the material [in] amusing and engaging ways". In March that same year, he began playing Tom Andrews on the Apple TV+ drama Pachinko. Chronicling an immigrant Korean family across four generations, the series was universally praised.
Simpson received positive notices for his portrayal of an alcoholic father in the 2023 coming-of-age film The Starling Girl, an independent drama about fundamentalist Christianity that Peter Debruge of Variety felt was "refreshing" and "rigorously realistic". From 2023 to 2024, he voiced Doctor Royce Hemlock in seasons 2 and 3 of the Disney+ animated series Star Wars: The Bad Batch.
Simpson wrote the upcoming horror comedy film Slay.
