McKinnon was originally set to portray Holmes until she exited the series in February.
The Dropout is based on the ABC News podcast of the same name about Holmes and the rise and fall of her company, Theranos.
Holmes lost millions and was indicted by a federal jury in 2018 after falsely claiming to have devised blood tests that only required small amounts of blood.
Seyfried is also producing. Elizabeth Meriwether is serving as showrunner and executive producer. Liz Heldens, Liz Hannah, Katherine Pope, Rebecca Jarvis, Victoria Thompson and Taylor Dunn are also executive producing.
The podcast was hosted by Jarvis and produced by Thompson and Dunn.
Holmes recently had her wire fraud trial delayed through Aug. 31 because she is pregnant. The trial was originally set to begin in July 2020 but has been delayed multiple times due to the COVID-19 pandemic.