Mortimer, who starred in the Leonardo DiCaprio film Shutter Island, will reportedly play the female lead in the tentatively titled More As This Story Develops, according to Deadline.
Actress Marisa Tomei (The Wrestler) was previously in talks for the same role, but negotiations appear to have fallen through.
Jeff Daniels signed on to play the male lead in March.
The project is not the first television series from the Social Network scribe, who has previously created three shows: Sports Night, the critically-acclaimed West Wing and Studio 60 on the Sunset Strip.
Olivia Munn and Alison Pill are currently in the final stages of negotiations for roles on the HBO pilot as well.
More As This Story Develops tells the story of a cable news anchor (Daniels) and his crew.