He played in Major League Baseball (MLB) for the Boston Red Sox, Philadelphia Phillies, Arizona Diamondbacks, and Toronto Blue Jays.

The Red Sox obtained their supplemental pick, the 42nd overall, as compensation for losing free agent Pedro Martínez to the New York Mets.

While competing for the Portland Sea Dogs in 2007, Buchholz played in fifteen games, winning seven and losing two.

"[8] Before Buchholz' first major league start, Red Sox manager Terry Francona said: "This kid can come up and pitch a no-hitter, and he's going to go back down...

"[9] In only his second career major league start on September 1, 2007, he became the first Boston Red Sox rookie to throw a no-hitter when he blanked the Baltimore Orioles 10–0.

The first two were for Hideo Nomo and Derek Lowe (in which the final score was also 10–0); the fourth was Jon Lester's on May 19, 2008, against the Kansas City Royals, which was also the first no-hitter in major league play after Buchholz's.

Buchholz was left off the postseason roster as the Red Sox eventually won the 2007 World Series in a four-game sweep over the Colorado Rockies.

[16] Red Sox pitching coach John Farrell stated that Buchholz would most likely be in line for 180–190 innings in the 2008 season.

On July 11, he was recalled to Boston and was part of their regular pitching rotation, though he did not record a single win after his return.

On August 20, as the Red Sox were trying to sweep the Orioles, the team that Buchholz had no-hit the preceding year, he gave up a 4–0 lead by allowing three runs in the second inning and two more in the third.

Buchholz stated "I've never had a streak like this", and "I've never been one to say the pressure was too much for me, but I've felt like I've had a lot of weight on my shoulders just trying to be perfect and trying to do everything as well as I could to help this team win".

[20] In his eighth start at Pawtucket in 2009, Buchholz took a perfect game into the ninth inning against the Louisville Bats, but it was denied after a leadoff single; ultimately, he pitched a one-hit shutout.

Tim Wakefield, Josh Beckett and Lester were the only dependable starters in the first half (John Smoltz, Daisuke Matsuzaka and Brad Penny all had losing records).

While Lester remained effective in the second half, Beckett began to struggle, and when All-Star Tim Wakefield was placed on the disabled list, Buchholz replaced him in the rotation.

Buchholz' pitching down the stretch played a large role in helping the Red Sox make it into the playoffs (along with a strong return from Matsuzaka).

He made his first start after his stint on the DL on July 21 against Gio González of the Oakland Athletics, pitching four innings, giving up five earned runs, with two strikeouts and three walks in a 6–4 loss.

Buchholz returned to his pre-injury form after the start in Oakland by going 4–0 in August with a 1.03 ERA and winning the AL pitcher of the month award.

[24] Buchholz finished sixth in the voting for the American League Cy Young Award, also won by Hernandez.

On April 14, Buchholz took a no-hitter into the top of the eighth inning against the Tampa Bay Rays before allowing a hit.

In the postseason, Buchholz made four starts despite re-aggravating his shoulder injury and won the World Series for the first time in his career when the Red Sox defeated the St. Louis Cardinals in six games.

Buchholz made his first start on April 6 against the Cleveland Indians, receiving a no decision after giving up five runs in four innings.

[35] Buchholz next pitched on July 21, and made a total of eight relief appearances before returning to the rotation for three starts in August.

For the season, Buchholz made a total of 37 appearances, including 21 starts, with an ERA of 4.78 in 139+1⁄3 innings pitched, and a win–loss record of 8–10.

[36] On December 20, 2016, Buchholz was traded to the Philadelphia Phillies for minor league second baseman Josh Tobias.

[39] On April 18, Buchholz underwent surgery on his right forearm to repair the flexor tendon, a procedure expected to require four to six months for recovery.

[41] He recorded a 1–0 mark, with nine strikeouts, in three appearances for the Double-A Northwest Arkansas Naturals and the Triple-A Omaha Storm Chasers.

Two weeks after signing with the Diamondbacks, manager Torey Lovullo announced that Buchholz would start on May 20, against the New York Mets.

[45] On August 16, he pitched a complete-game against the San Diego Padres, yielding only a solo home run to Hunter Renfroe.

He partnered with former teammate Jon Lester, who also released a Charity Wine, called CabernAce, to raise money for the Fred Hutchinson Cancer Research Center.

[54] In 2020, Buchholz and his wife filed a lawsuit against Crestbrook Insurance Company after their Austin, Texas, home was demolished due to the presence of mold, with the couple claiming the insurance company refused to cover the cost of damages, that their infant son needed medical attention after sleeping in a mold-infested room, and that they refused to relocate the family to a temporary home.