Their casting was revealed by TVLine, which also confirmed that the show's US premiere date is now officially Sunday, April 18.
Fans of the network's Watchmen series will recognise Smart for her commanding role as Agent Laurie Blake, while the 69-year-old actress boasts recent credits in Fargo's second season and Legion, too.
She's onboard as Winslet's character's mother, Helen.
The 34-year-old will play a detective in Mare of Easttown named Colin Zabel.
Created by Out of the Furnace's Brad Ingelsby, this seven-episode drama follows Winslet's Mare Sheehan - "a small-town Pennsylvania detective who investigates a local murder as life crumbles around her."
HBO goes on to describe the project as "exploration into the dark side of a close community, and an authentic examination of how family and past tragedies can define our present."
Mare of Easttown comes to HBO on April 18, while its UK premiere date is yet to be confirmed.
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