Cherington, who was most recently the Toronto Blue Jays' vice president of baseball operations, was seen in Pittsburgh on Friday, according to Mackey.
It's expected that Cherington will be officially introduced on Monday, according to Adam Berry of MLB.com. Details of Cherington's contract with the team aren't yet known.
The 45-year-old spent over a decade working in multiple roles with the Boston Red Sox, including a stint as their general manager from 2011 to 2015. He was the architect of Boston's 2013 World Series championship club, and he helped draft, sign, and develop the current crop of Red Sox stars including Xander Bogaerts, Mookie Betts, and Andrew Benintendi.
Cherington left the Red Sox in August 2015, shortly after the team hired Dave Dombrowski as president. He joined the Blue Jays' front office one year later.
Cherington replaces longtime GM Neal Huntington, who the Pirates fired last month after 12 seasons.
The Pirates, who lost 93 games in 2019, are the only team in MLB without a field manager following Clint Hurdle's dismissal in September. Pittsburgh's search for Hurdle's replacement had been temporarily paused following Huntington's firing, according to Jon Heyman of MLB Network.
While the Cherington hiring hasn't been made official, the Pirates did announce Friday that assistant GM Kyle Stark has been fired, according to Mark Feinsand of MLB.com.