By the age of 13, he had begun training with Colorado baseball guru Bus Campbell, who had helped almost every promising pitcher from the Denver area, including Goose Gossage and Brad Lidge.

[15] In his second career start, against the Detroit Tigers on September 27, 1998, Halladay had what would have been the third no-hitter ever pitched on the final day of a regular season broken up with two outs in the ninth.

The feat would have joined the combined no-hitter by four Oakland Athletics pitchers (Vida Blue, Glenn Abbott, Paul Lindblad, and Rollie Fingers) in 1975 and Mike Witt's perfect game in 1984.

The bid was broken up by pinch hitter Bobby Higginson's solo home run, the only hit allowed in a 2–1 Toronto victory, as Halladay recorded his first major league win.

[16][17] Prior to the home run, the sole base runner had reached on an infield error in the fifth inning, as Halladay struck out eight and walked none.

[18] Halladay pitched the first extra-inning shutout in the major leagues since Jack Morris in Game 7 of the 1991 World Series, leading the Blue Jays to victory over the Tigers on September 6.

[24] Halladay won the American League Cy Young Award, while being once again named an All-Star and leading the Blue Jays to a surprising 86 victories.

Although Halladay's strikeout total was lower in 2006 than in previous seasons, his ground ball/fly ball ratio, complete games, and innings pitched were all among the American League leaders.

Against the Los Angeles Dodgers on June 10, his ground ball single to center field allowed John McDonald to score.

[30] Although he was given a clean bill of health for his next start, it was later suggested by television commentators that Halladay may have in fact suffered a temporary lapse in recognition of what happened on the play.

[44] On December 15, 2009, the Blue Jays traded Halladay to the Philadelphia Phillies for minor league prospects Travis d'Arnaud, Kyle Drabek, and Michael Taylor.

[46][47] In fact, the Phillies' acquisition of Halladay and trade of Lee were deeply intertwined, as the team wanted to strengthen the rotation without emptying the farm system of prospects.

[55] The Phillies swept the Reds in three games to advance to their third consecutive National League Championship Series, where they faced the San Francisco Giants.

The Philadelphia chapter of the Baseball Writers' Association of America presented him with the "Steve Carlton Most Valuable Pitcher" and "Dallas Green Special Achievement" awards.

[67] In 250+2⁄3 innings pitched, Halladay finished the 2010 regular season with a 21–10 record and a 2.44 ERA, setting a career high with 219 strikeouts while issuing just 30 walks.

[68] On May 29, 2010, Halladay pitched the 20th perfect game in MLB history against the Florida Marlins in Miami, retiring all 27 batters and striking out 11, allowing no hits, runs, walks, or errors.

[70] On October 6, 2010, in his first postseason appearance, Halladay pitched a no-hitter (his second of the season), against the Cincinnati Reds in the first game of the National League Division Series (NLDS).

[71] This also marked the first time in Major League history that a pitcher threw a perfect game and another no-hitter in the same calendar year (including the postseason).

The resulting starting pitching lineup of Halladay, Lee, Cole Hamels, Roy Oswalt and Joe Blanton had commentators dub it one of the best rotations ever assembled.

[57] This loss eliminated the Phillies from the playoffs, a disappointment as they were touted as heavy favorites for the World Series,[85] and it would turn out to be Halladay's final postseason appearance.

[90] On April 5, 2012, Halladay threw eight innings of shutout ball against the Pittsburgh Pirates on Opening Day, giving up two hits while striking out five.

[92] In a press conference on June 6, Halladay stated, "Ultimately, my goal is to finish my career with the Phillies and win a World Series here.

[98] After struggling in spring training,[99] Halladay gave up five runs in his first start in the Phillies second game on April 3, 2013, striking out nine in 3+1⁄3 innings pitched.

[18] On December 9, 2013, Halladay signed a ceremonial one-day contract with the Blue Jays and announced his retirement from baseball due to injury.

[109] Despite his reputation as a ground ball pitcher who worked at an efficient pace, Halladay's strikeout totals increased steadily in the few final years of his career.

[57] The Pasco Sheriff's Office Marine Unit responded to the accident after a call at noon, reporting that a sport plane had crashed upside-down into shallow water.

[121][122] An autopsy report by the Pinellas-Pasco Medical Examiner's Office released in January 2018 revealed that Halladay's blood contained morphine, hydromorphone, amphetamine, fluoxetine (an antidepressant), baclofen (a muscle relaxant), and zolpidem (a sleep aid sold under the brand name Ambien).

"[123] On April 15, 2020, the National Transportation Safety Board (NTSB) released a report stating that in addition to the drugs, Halladay was executing aerobatics including steep climbs and turns, and the plane sometimes came within 5 feet (1.5 m) of the water before a final climb caused its speed to fall to 85 mph (137 km/h), after which it nosedived into the sea and Halladay was killed by blunt force trauma and drowning.

[124][125] The NTSB determined the probable cause of the crash to be the "pilot's improper decision to perform aggressive, low-altitude maneuvers due to his impairment from the use of multiple psychoactive substances, which resulted in a loss of control".

His wife and sons announced that they did not choose a logo for his cap, which leaves Roberto Alomar as the sole Cooperstown inductee as a Blue Jay.