Mark Zuckerman of MASN Sports first reported the news.
On Tuesday, Nationals general manager Mike Rizzo provided an update, per Jesse Dougherty of the Washington Post:
Bench coach Chip Hale took over the managing duties when Martinez left Sunday's contest in the sixth inning and will do so again during Monday's game, per the team.
Washington hired Martinez, who turns 55 on Sept. 26, as its manager prior to the 2018 season. He spent 16 years in the major leagues as an outfielder and had stints as the bench coach for the Tampa Bay Rays and Chicago Cubs.
He was part of the coaching staff that helped lead the Cubs to their first World Series title in 108 years during the 2016 season.
The Nationals missed the playoffs at 82-80 in their first season under Martinez but are in position to reach the postseason in 2019 despite losing Bryce Harper in free agency. They are 82-66 and 1.5 games ahead of Chicago for the top wild-card spot in the National League.
The series against the Cardinals could prove important for wild-card positioning since St. Louis is only two games ahead of the Cubs in the NL Central.