Amid a fairly quiet off-season on the trade front, the Nationals made a deal with the division-rival New York Mets shortly before opening their spring training camp to acquire utility infielder Matt Reynolds for cash considerations.

[75] The Nationals announced 21 non-roster invitees to major league spring training on February 13, 2018: pitchers Tim Collins, Jimmy Cordero, Brady Dragmire, David Goforth, Ismael Guillón, Bryan Harper, Edwin Jackson, Jaron Long, Román Méndez, Tommy Milone, Chris Smith, and César Vargas; catchers Taylor Gushue, Spencer Kieboom, Miguel Montero, and Jhonatan Solano; infielders Osvaldo Abreu, Reid Brignac, and Chris Dominguez; and outfielders Ryan Raburn and Moises Sierra.

[79] Following the Nationals' signing of Joaquin Benoit to contend for a spot in the major league bullpen, starting pitcher Joe Ross, still rehabbing from Tommy John surgery in July 2017, was placed on the 60-day disabled list effective February 21.

The Nationals challenged Mets starter Jacob deGrom in the sixth, loading the bases with no outs, but failed to score, and Washington shortstop Trea Turner argued with umpire Doug Eddings over an inning-ending called strike and was ejected from the game[121] for the first time in his career.

[159] On May 1, for the second game of the Pittsburgh series, manager Dave Martinez decided to shake up the lineup by having right fielder Bryce Harper bat leadoff for the first time since 2013, hoping that it would give more at-bats and force opposing pitchers to pitch to him instead of routine walking him.

With additional offense from left fielder Matt Adams and shortstop Trea Turner drove in three runs, the Nationals jumped out to a 12–2 lead behind another strong outing by starter Max Scherzer and hung on for a 12–4 win.

[160] The streak stretched to four, marking the first time they had had four consecutive victories since the first four games of the season, as starter Stephen Strasburg led the Nationals to a third win over the Pirates on May 2 with an 11-strikeout performance backed by Harper, Adams, and second baseman Howie Kendrick.

[177] The Nationals experienced an unusually long layoff as a two-game homestand against the New York Yankees scheduled to start May 15 was rained out; the teams were able to begin the first game, which was suspended at a 3–3 tie midway through the fifth inning, but it was ultimately continued to June 18 as inclement weather prevented any further action.

[180] After placing Kendrick on the disabled list in between games,[181] the Nationals recalled infielder Adrián Sánchez from Class-AAA Syracuse as the 26th man,[182] as well as right-handed pitcher Jefry Rodríguez from the Class-AA Harrisburg Senators in his first major league posting.

[186] The three-game skid marked the Nationals' worst of the month, a disappointment exacerbated as the team lost another key player, setup man Ryan Madson, to the disabled list with a pectoral strain.

[188] The Nationals went on to thrash the Padres 10–2, then walked off over the visitors in the ninth inning 2–1 the following day as center fielder Michael A. Taylor doubled home Soto, who reached base in all four of his plate appearances,[note 9] for the winning run.

[189] However, they dropped the final game of the three-game series on May 23 as spot starter Erick Fedde, recalled for the day from Class-AAA Syracuse for his season debut and fourth career start, was outdueled by San Diego ace Tyson Ross.

[note 11] With Rays reliever Jonny Venters making his first career start on June 6, the Nationals hammered him for five runs in 1⁄3 innings en route to an 11–2 win and a sweep of the two-game set, with third baseman Anthony Rendon celebrating his 28th birthday with a four-hit performance.

[211] Washington suffered its second blow to the rotation in less than a week's time on June 8, as starter Stephen Strasburg struggled to a loss and exited after two innings with right shoulder discomfort in a home game against the San Francisco Giants.

But even with Murphy and Eaton both in the lineup for the first time since April 2017, the Nationals' offensive woes continued as Yankees starter CC Sabathia led his team in a 3–0 blanking of the visitors despite a quality start turned in by Roark.

[239] The home team stormed back from a four-run deficit with six unanswered runs in a hitting outburst keyed by Harper and Murphy, both of whom had been struggling at the plate throughout the month of June, along with Turner, Rendon, Soto, and center fielder Michael A. Taylor.

[251] The Independence Day game ended with Matt Adams, who had been activated from the disabled list that morning (reliever Tim Collins was designated for assignment in a corresponding move), standing in the on-base circle to pinch-hit as the potential go-ahead run as Boston closer Craig Kimbrel induced a popout from second baseman Wilmer Difo.

[253] Fedde was placed on the 10-day disabled list with what the team called right shoulder inflammation on July 5, and right-handed pitcher Jefry Rodríguez was recalled from the Class-AAA Syracuse Chiefs to replace him on the roster.

[260] The Marlins won 10–2, with Martinez finally inserting a position player in to pitch in the ninth inning: Reynolds, who made his major league debut as a pitcher after starting at third base (to which catcher Spencer Kieboom switched for the first time in his professional career) and retired the only batter he faced on a groundout to the first baseman Adams.

[307][308] Speaking both of the decision to designate Kelley, whose behavior he called "selfish" and "disrespectful" toward Martinez, and trade former Nationals reliever Brandon Kintzler to the Chicago Cubs the day before, Rizzo declared, "If you're not in, you're in the way.

"[309] The Nationals selected the contract of right-handed relief prospect Jimmy Cordero to fill the roster spot vacated by Kelley,[310] who was traded days later to the Oakland Athletics for international bonus slot money.

The team recalled right-handed reliever Koda Glover from Syracuse to replace Herrera on the roster, announcing erstwhile setup man Ryan Madson would take over closing duties temporarily.

Instead, after loading the bases via an infield single bobbled by defensive substitute Wilmer Difo and two hits-by-pitch,[328] Madson surrendered a two-out, two-strike walk-off grand slam to rookie pinch-hitter David Bote.

[note 27] After the game, Madson said he had been experiencing shooting pain in his back and right leg and had been unable to grip the ball well enough to throw his signature curveball, but he had not disclosed his condition to the coaching staff previously.

[333] Before the second game of the series on August 14, the Nationals removed both Madson and Solís from their active roster, placing the former on the 10-day disabled list with what the team called "lumbar nerve root irritation" and optioning the latter to Class-AAA Syracuse.

[346][347][348][349][350][351][352] Despite losing the veteran infielders, who were claimed off waivers, the Nationals rode key hits by outfielder Andrew Stevenson, recalled from Class-AAA Syracuse in a related move, and second baseman Wilmer Difo, assuming everyday duties from Murphy, to win on August 21 in the opening game of a series against the division-rival Philadelphia Phillies.

[353] The following night, the Nationals overcame the struggles of just-reactivated starter Stephen Strasburg and bounced back from a one-run deficit to stun the Phillies on first baseman Ryan Zimmerman's eleventh career walk-off home run, a bomb to right field that scored Soto from second base.

Trailing by two runs when the rain delay started in the eighth inning, the Nationals rallied behind two-out RBI singles from shortstop Trea Turner and rookie left fielder Juan Soto to take the lead, with reliever Justin Miller locking down the save to complete the come-from-behind victory.

Trailing into the ninth inning, center fielder Bryce Harper led the team to a dramatic comeback with a two-run home run off St. Louis Cardinals closer Bud Norris to tie the game followed by a tenth-inning sacrifice fly off his former College of Southern Nevada teammate Chasen Shreve.

From May 25 to 27, he made his broadcasting debut by filling in for Ray Knight as the studio analyst on the Nats Xtra pregame and postgame shows on MASN, working with host Johnny Holliday during a weekend three-game Washington Nationals series at the Miami Marlins.