To make room for Maldonado on the 40-man roster, the team also transferred catcher Wilson Ramos to the 60-day disabled list.
This is Maldonado's second Major League stint with the Nationals -- the first coming in 2010. He joins the team a day after Leon was injured in a home-plate collision against the Padres.
It marked the second injury by a Nationals catcher in the last three days. On Saturday, Ramos tore his ACL while going after a passed ball against the Reds. He is expected to be out for the season.
Maldonado, 33, was on his way to Columbus, Ohio, around 11 p.m. ET when he received word that he was promoted to the big leagues. At first, he thought his teammates were pulling a prank on him. But soon thereafter, one of his teammates showed him the video of Leon getting hurt. Maldonado then boarded a plane from Columbus at 7:30 a.m. on Tuesday and flew to Washington.
"It's crazy, what's going on. My teammates [in Syracuse] were making fun of me, [telling me to leave] the catching gear all the time to make sure I don't get hurt," Maldonado joked.
But Maldonado was serious when he talked about Leon and Ramos. All three are from Venezuela, and he was shocked to learned about the injuries they suffered.
"They are from Venezuela, and I played winter ball with Sandy," Maldonado said. "We talk a lot. It's sad what happened to them. I'm excited to get called up, but in the meantime, it's sad, because it was the first time Sandy was here. It was the first game he played [in the big leagues], and I feel bad for him."
Maldonado, who will wear No. 49, was hitting .211 with two home runs and six RBIs in 21 contests this season with Syracuse.