Thornton is in the first season of a two-year, $7 million contract, and the Nationals will assume the remainder of the contract. He is 0-3 with a 2.55 ERA in 24 2/3 innings, and lefties have hit .250 against him.
The Yankees drafted lefty reliever Jacob Lindgren out of Mississippi State with their first pick in June's draft and have promoted him to Double-A. He could be fast-tracked to the majors by the end of the month. Manager Joe Girardi also mentioned young left-handed relievers Tyler Webb and James Pazos in explaining why the Yankees felt Thornton was expendable.
In the short term, the Yankees called up veteran lefty Rich Hill, who had been pitching at Triple-A Scranton/Wilkes-Barre.
Yankees general manager Brian Cashman added money when he traded for utilityman Martin Prado. Cashman said he went to ownership to ask for more payroll to fit Prado's deal, which calls for $11 million for next season. The Yankees will save around $4 million by trading Thornton.
The club also transferred right-hander Taylor Jordan to the 60-day disabled list, making room on the 40-man roster for Thornton.
Ramos and his wife are expecting their first child. Ramos is hitting .290 with three homers and 25 RBIs in 49 games this season.
Leon rejoins the Nationals for the third time this season.
ESPNNewYork.com contributor Danny Knobler and The Associated Press contributed to this story.