On February 7, 2019, the Kansas City Royals signed Boxberger to a 1-year contract for $2.2 million, with $1 million in performance bonuses. Boxberger was designated for assignment on June 26 and released on July 3.
Boxberger signed a minor-league deal with the Washington Nationals on July 12, 2019. He was assigned to the Nationals' Class-AA affiliate, the Harrisburg Senators. Boxberger was released on August 4, 2019, after pitching to a 1.04 ERA over eight games out of the Senators' bullpen.
On August 5, 2019, it was announced that the Cincinnati Reds had signed Boxberger to a minor league deal and optioned him to the Triple-A Louisville Bats. Boxberger was released by the Reds organization on August 23, 2019.