[4] On January 22, 2015, the Nationals signed right-handed reliever Evan Meek and utility man Kila Ka'aihue to minor-league deals with invitations to spring training.
but a pair of errors by the shortstop Ian Desmond in the sixth and seventh innings allowed the Mets to score three runs, causing the Nationals to lose the game 3–1.
The team lost on Opening Day to the New York Mets, with Max Scherzer getting his first regular-season start as a Washington National and taking the loss despite pitching well, with shortstop Ian Desmond and second baseman Dan Uggla contributing to the defeat with defensive miscues in the middle infield.
Right-handed relief pitcher Craig Stammen tore a flexor tendon in his pitching arm and needed surgery to repair it, abruptly ending his 2015 campaign just a couple weeks into the season.
[27] Left-hander Felipe Rivero, called up to add depth to the bullpen, only made one appearance in a loss to the Phillies before going on the disabled list with gastrointestinal bleeding.
[41][42] Rookie outfielder Michael Taylor, who subbed in for an ejected Bryce Harper in a May 13 matchup against the Diamondbacks, hit his first career grand slam off Addison Reed in the top of the ninth inning to win the game for his team.
[43] Wilmer Difo, an infielder who started the year at the team's High-A affiliate Potomac Nationals, was called up on May 19 and hit a single up the middle against the New York Yankees on his first-ever major-league at-bat.
[45] The second ejection, by veteran umpire Marvin Hudson, faced criticism from The Washington Post and the Nationals, with Williams saying after the game, "I can't explain it.
[38] After being hit by a pitch by Padres starter Odrisamer Despaigne on May 15, veteran outfielder Jayson Werth was discovered to have two fractures in his wrist, keeping him sidelined until August at the earliest.
[47] Stephen Strasburg pitched poorly in several games and was often pulled out early, until the team finally placed him on the disabled list following a short start against the Cincinnati Reds on May 29 due to neck tightness.
[50][51] The Nationals' only left-handed starting pitcher, Gio González, was hit by pitches twice at the plate in the May 30 game, including once in his throwing arm, which led to him getting pulled earlier than planned after surrendering two runs when he next took the mound.
[51] Center fielder Denard Span hurt his knee making a play in the outfield in the last game on May 31, putting him on a day-to-day injury watch as well.
[52] Grace made a disastrous relief appearance in the last game of the Reds series, surrendering four earned runs while failing to retire any of the five batters he faced before he was pulled.
[57] With Doug Fister and Stephen Strasburg still dealing with nagging injuries, the Nationals called up Joe Ross, a right-handed pitcher, from the Double-A Harrisburg Senators to temporarily join the rotation.
[59] Ross notched his first major league win on June 13, besting Jimmy Nelson and the Milwaukee Brewers at Miller Park in a 7–2 game.
Scherzer also became only the third Major League Baseball pitcher in history to pitch a one-hit shutout with 16 or more strikeouts and one walk or less, joining Nolan Ryan and Kerry Wood.
It was the longest streak of consecutive scoreless innings by starting pitchers in the history of the Montreal–Washington franchise, and the second-longest by any Major League Baseball team since the beginning of the "expansion era" in 1961.
He became the seventeenth major-league pitcher to receive the honor in two consecutive months and the first since Clayton Kershaw of the Los Angeles Dodgers did it in June and July 2014.
Star outfielder Bryce Harper did play in the All-Star Game after finishing number one overall in fan votes received, a new record at more than 13.9 million, though he went hitless.
Center fielder Denard Span was placed on the disabled list effective July 7 after attempting to play through persistent back spasms and skipping a few games to rest.
[76] Newly acquired right-handed reliever David Carpenter landed on the disabled list with inflammation in his throwing shoulder on July 12, shortly after returning from paternity leave.
[82] He was joined on July 28 by outfielder Jayson Werth and first baseman Ryan Zimmerman, who missed 61 and 49 games respectively in lengthy disabled list stints.
[83] Expecting a big boost from the return of their previously injured players,[84] the Nationals were less active than their main division rivals, the Mets, at the trade deadline.
However, they made a major move by trading with another division rival team, the Philadelphia Phillies, for closer Jonathan Papelbon in exchange for minor league starting pitcher Nick Pivetta.
[86] The Nationals continued to lose ground to the New York Mets, who swept them in a series at Citi Field in Queens and then retook sole possession of the division lead on August 4.
[95] Fellow veteran Ryan Zimmerman, the Nationals' starting first baseman, fared better in his return from the disabled list, batting in 28 runs in August to tie a team record.
[105] After allowing the eventual game-winning home run to Yoenis Céspedes on September 9,[104] reliever Drew Storen slammed his hand on a clubhouse locker and broke his thumb, bringing an end to his season.
[106] First baseman Ryan Zimmerman was also forced to sit out the rest of the season due to injury after straining an oblique muscle in the midst of an 11-game hitting streak with six home runs.
[113] With Drew Storen injured and Jonathan Papelbon suspended, rookie left-hander Felipe Rivero assumed the role of closer for the last week of the season, recording his first career save against the Braves on October 1.
In accordance with the agreement, the Nationals announced midway through the 2015 season that a likeness of Calvin Coolidge ("Cal" for short), the 30th President of the United States, would become the sixth Racing President, as well as the first one to enter the race while an MLB season was in progress, joining George Washington ("George"), Thomas Jefferson ("Tom"), Abraham Lincoln ("Abe"), Theodore Roosevelt ("Teddy"), and William Howard Taft ("Bill").