Roy Halladay's rookie season in 1999 featured 3.92 ERA in 149 1⁄3 innings for the Toronto Blue Jays. In his second year, he finished with an ERA of 10.64, the worst by any pitcher who tossed 50 innings or more. He improved from this dismal performance, and he played 12 seasons for the Toronto Blue Jays, in which time he averaged 17 wins per season, made six all-star teams and, in 2003, won a Cy Young Award. In his second major league start, on September 27, 1998, he retired 26 Detroit Tigers in a row before he gave up a home run to Bobby Higginson.
On Opening Day, Halladay pitched seven innings while giving up a run against the Washington Nationals in his first game with the Phillies. He had nine strikeouts and allowed six hits. He also drove in his second career RBI and earned his first win of the season. He followed this start with a complete game on April 11 against the Houston Astros, giving up one unearned run while striking out eight and not giving up any walks in the Phillies' 2–1 victory.
Halladay pitched his first shutout in the National League, against the Atlanta Braves on April 21, becoming the first pitcher to reach four wins in the 2010 season. On May 1, Halladay pitched his second shutout of the season, limiting division rival New York Mets to three hits and striking out six.
The Philadelphia Phillies–Florida Marlins game began at 7:13 p.m. on May 29, 2010, to a crowd of 25,086 at Sun Life Stadium in Miami. The home plate umpire was Mike DiMuro, the first base umpire was Tim Welke, the second base umpire was Jim Reynolds, and the third base umpire was Bill Welke. Roy Halladay pitched the 20th perfect game in MLB history, retiring all 27 batters, and allowing no hits, no runs, and no errors. Although he is known as a ground ball pitcher, he struck out 11, his most of the year and the most by a Phillie pitcher at that point in the season. The opposing pitcher, Josh Johnson, allowed seven hits, one walk, and an unearned run in seven innings. The unearned run came in the top of the third, when Chase Utley's line drive bounced off the glove of Florida center fielder Cameron Maybin, allowing Wilson Valdez to score from first. This was the deciding factor in the game, for Josh Johnson did not allow another run for the duration of the game.
Halladay's first pitch came at 7:17 pm, a called strike to Chris Coghlan off a 92 m.p.h. fastball on the outside corner. He threw a total of 19 pitches in the first inning, his most of the night, and threw no more than 12 pitches in any following inning other than the seventh. In total, he threw 115 pitches, including 72 for strikes. Halladay worked his way out of seven three-ball counts and six full counts, and credited the effectiveness of his sinker for his success. Of his 11 strikeouts, four came by way of sinkers.
In the bottom of the ninth, pinch-hitter Mike Lamb flied out deep to Shane Victorino in center field for the first out. The ball would have been a home run in many other ballparks, but did not carry out of Sun Life Stadium's cavernous center field. Halladay thought he had lost the perfect game when the ball was initially struck. Victorino later commented on flagging the ball down, saying "I was going to do whatever it took." For the second out, Wes Helms, another pinch-hitter, struck out looking – the sixth batter to do so. In the game-ending play, Ronnie Paulino was thrown a 1-2 curveball and hit a hard chopper to Juan Castro at third base; Castro reached wide to his left to grab the ball, spun and threw to first base to preserve the perfect game. It was the second such play made by Castro in the game. Castro had been chosen to start at third base over Greg Dobbs at the start of the game due to Dobbs's two-run error in Halladay's previous start.
Halladay praised catcher Carlos Ruiz, saying, "I can't say enough about the job that Ruiz did tonight, really. I felt like he was calling a great game up until the fourth or fifth, and at that point, I just felt like I'd let him take over and go with him. He did a great job. Like I said, it was kind of a no-brainer for me. I'd just go out, see the glove and hit it." Halladay only shrugged off Ruiz once the entire game.
Halladay became the 10th pitcher in Phillies history to hurl a no-hitter; Jim Bunning's perfect game in 1964 was the only other time a Phillie pitcher tossed a perfect game. In addition, this was the eighth perfect game in National League history, the last being Randy Johnson's in 2004. This was the first time in the modern era that two pitchers—Dallas Braden and Halladay—had thrown perfect games in the same month and that multiple perfect games had been achieved in the same season. The twenty days between Braden's perfect game and Halladay's was the shortest span between two perfect games since 1880.
Unfortunately, it came at a bad timing for the Phillies and for Philadelphia sports fans; Halladay pitched his perfect game on the same night the Philadelphia Flyers played the Chicago Blackhawks in Game 1 of the Stanley Cup Finals. The opening face-off at the United Center in Chicago took place an hour after the Phillies-Marlins game began, resulting in many Phillies fans, including fans and media there, missing the perfect game in lieu of the hockey game, which the Flyers lost, 6–5.
The Phillies's play-by-play man Tom McCarthy did the television broadcast, on Comcast SportsNet Philadelphia. He did what he could to uphold the tradition of not mentioning a perfect game until it was over, frequently alluding to the fact that Halladay was "pitching a masterpiece", but he broke the tradition in the 9th inning, mentioning the perfect game right before the 27th batter. Scott Franzke did the radio broadcast of the game for WPHT-AM.
Many Phillies fans were following the game between the Flyers and the Blackhawks on NBC. They came to know about the perfect game from Pierre McGuire, Mike Milbury, and Ryan Miller when the network replayed the final out during the first intermission report. As a result of the timing, Comcast SportsNet Philadelphia decided to rebroadcast the game two nights later, though it happened on the same day the Flyers played Game 2 of the Stanley Cup Finals.
For the Marlins, Rich Waltz called the play on FS Florida, while radio broadcaster Glenn Geffner called the play on WAXY, the Marlins radio station.
When Halladay's former manager, Cito Gaston, called to congratulate him, Halladay was unable to take the call because he was busy with the post-game media frenzy. After the end of the frenzy, he worked out, like he does after every pitching start. Vice-President Joe Biden, an avid Phillies fan, called Halladay to congratulate him as well.
On June 2, 2010, Armando Galarraga of the Detroit Tigers retired the first 26 batters before the 27th batter reached base on an errant call by first base umpire Jim Joyce. Had Galarraga's game been correctly called, the four-day span since Halladay's perfect game would have broken that 130-year-old record, and marked the only time that three consecutive no-hitters had been perfect games, the only time that three perfect games had occurred in one season, the only time that three perfect games had occurred in a shorter span than a month, the only time four perfect games had occurred in a span shorter than a year, and the only time that four perfect games had occurred within a stretch of five no-hitters.
On August 24, 2010, to commemorate his perfect game, Halladay presented around 60 Swiss-made Baume and Mercier watches he had purchased to everyone in the clubhouse. The watches were presented in brown boxes that bore the inscription: "We did it together. Thanks, Roy Halladay." Additionally, the back of each watch was engraved with the date of the game, the line score, and the individual recipient's name.
In his first career postseason start on October 6, Halladay hurled a no-hitter, giving up only one walk (to Jay Bruce of the Cincinnati Reds) in the fifth inning of Game 1 of the NLDS. Halladay's was only the second postseason no-hitter in Major League Baseball history, and the first since Don Larsen's perfect game in the 1956 World Series. He threw only 104 pitches. He thus became the only pitcher to throw a no-hitter in the regular season and a no-hitter in the postseason in the same year. Halladay is also the fifth major league pitcher to throw two no-hitters in the same year, and the first since Nolan Ryan in 1973. This was the first no-hitter to take place at Citizens Bank Park, which opened in 2004.