As the Eagles' reigning staff ace, Iwakuma won the Eiji Sawamura Award in 2008, and was also a NPB All-Star that year.

On August 12, 2015, Iwakuma became the second Japanese player ever to pitch a no-hitter in Major League Baseball (joining Hideo Nomo, who threw two), throwing a three-walk, seven-strikeout 3–0 win over the Baltimore Orioles at Safeco Field.

Iwakuma was born in Higashiyamato, Tokyo, and began playing baseball in the first grade before attending Horikoshi High School in Nakano.

[7] Iwakuma was named the Eagles' starter for the 2005 season opener, holding the Chiba Lotte Marines to one run while going the distance on March 26 and earning the expansion team's first-ever win.

[8] However, while he did not miss a single start during the regular season, he was bothered by tenderness in his shoulder throughout the year, finishing with a 9-15 record and an ERA of 4.99 (worst among all qualifying pitchers).

[10] Chosen to start the season opener for the second straight year (and fourth time overall), Iwakuma pitched against the Fukuoka SoftBank Hawks on March 20, 2008, but fell short of the win despite limiting the Hawks to just one run over seven innings (then-closer Domingo Guzmán gave up a walk-off home run to outfielder Hiroshi Shibahara, handing Iwakuma a no-decision in a 4–3 loss).

On September 22, in a game against the Saitama Seibu Lions, Iwakuma threw seven innings of one-run ball to become the first 20-game winner in the Pacific League since Hawks right-hander Kazumi Saito achieved the feat in 2003.

[11] He picked up one more win on October 5 against the Hawks, becoming the first 21-game winner in 23 years (former Hankyu Braves right-hander Yoshinori Satoh won 21 games in 1985) and passing young rival and fellow ace Yu Darvish of the Fighters to lead the league in ERA (1.87) in his final start of the season.

[12] He gave up just three home runs in 2012⁄3 innings all year, of which just one was to a Pacific League hitter (the other two were given up in interleague games), and won nearly one-third of the Eagles' 65 wins in the regular season himself.

[13] Iwakuma started the season opener for the Eagles for the third straight year in 2009, requiring just 59 pitches to throw six innings of one-run ball en route to the win on April 3 in a much-hyped match-up with World Baseball Classic teammate and 22-year-old Fighters right-hander Yu Darvish (who threw 121 pitches in a complete game loss, all three runs given up in the first inning).

[15] On October 4, 2010, the Golden Eagles granted permission for Iwakuma to pursue a career in Major League Baseball via the posting system.

On September 18, 2013, Iwakuma pitched eight scoreless innings against the Detroit Tigers striking out Miguel Cabrera twice making him 0 for 4 and lowering his batting average to .347.

Iwakuma finished third in the 2013 American League Cy Young Award voting, behind winner Max Scherzer and runner up Yu Darvish.

[33] Iwakuma made his first appearance on the international stage when he was named to the Japanese national team that would play in the 2004 Athens Olympics.

However, he allowed seven baserunners (three hits, three walks, and a hit-batter) and gave up three runs (two earned) against a team that was viewed as the heavy underdog, leading head coach (and acting manager) Kiyoshi Nakahata to pull him from the game after just 12⁄3 innings.

[34] Though Japan went on to win the game 8–3 behind a strong relief effort (five shutout innings) by future MLB pitcher Hiroki Kuroda, Iwakuma lost the trust of the coaching staff and did not pitch in the tournament again.

[36] His stellar effort convinced manager Tatsunori Hara to choose him over Darvish, who had been unofficially tabbed as the staff ace prior to the tournament, as the starter for Japan in the championship game against South Korea.

[37] While Darvish gave up the tying run in the ninth after coming on in relief, Iwakuma played an instrumental role in Japan's eventual win and second consecutive title.

He records a moderate number of strikeouts each season but is inherently a groundball pitcher, adept at jamming opposing hitters with his wide assortment of offspeed pitches and excellent command[43][44] (2.00 career walks per nine innings rate in NPB).

Iwakuma has also remarked that his command has improved since missing much of the 2006 and 2007 seasons due to injury as he was forced to overhaul his mechanics and rethink his approach against opposing batters.

[45] Iwakuma's wife, Madoka, and two children, Towa and Uta, moved with him from Japan to Seattle at the start of his Mariner career.