Crowe will lead the cast as the late Fox News boss, who died last year (17), and now Sienna has signed on to portray Roger's former wife, Beth Ailes.
Meanwhile, "Family Guy" creator Seth MacFarlane has signed on as the TV chief's confidante Brian Lewis, and "The Theory of Everything" actor Simon McBurney will play media mogul Rupert Murdoch, who hired Ailes to take charge of his news network in 1996.
Also joining the line-up is Annabelle Wallis, who will reunite with her "The Mummy" co-star Crowe when she portrays Fox News executive Laurie Luhn, who rose through the ranks at the network after embarking on an allegedly abusive relationship with Ailes.
Naomi Watts had previously been announced as U.S. newswoman Gretchen Carlson for the limited series, based on Gabriel Sherman's bestselling book "The Loudest Voice in the Room".
Ailes was ousted from Fox News in 2016, after facing a series of sexual misconduct allegations, and he went on to serve as an advisor for the Donald Trump presidential campaign.
The as-yet-untitled series focuses on the last 10 years of Ailes' life and airs on U.S. network Showtime.
The TV project is being developed at the same time as director Jay Roach's planned Ailes biopic. The film also boasts an all-star line-up, with John Lithgow as Ailes, alongside Nicole Kidman, Margot Robbie, Charlize Theron, and Malcolm McDowell as Murdoch.