Johnson converted all nine of his save opportunities in May. He is tied for the Major League lead with 17 this season, and he has 25 straight conversions since Aug. 14, 2011.
"The mark of a true closer, you know you've got a really good one when they can close a one-run game on the road against the American League East," manager Buck Showalter said. "And Jimmy's done that for us this year."
The 28-year-old righty pitched 12 games in May, sporting a 1.42 ERA. In 12 2/3 innings, he held the opposition to seven hits, struck out eight and didn't walk a batter.
His month included saves on three straight days from May 10-12 and two scoreless innings with three strikeouts during the Orioles' 17-inning win over the Red Sox on May 6 at Fenway Park.
Entering Sunday's action, Johnson held a 0.78 ERA in 23 innings, helping the Orioles into a tie for first in the American League East.
Andrew Simon is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.