Iwamura was born on February 9, 1979, in Uwajima, Japan, in Ehime Prefecture. He attended Uwajima Higashi High School. He played in the Philippines on the Japan National high school baseball team.

His older brother, Takashi Iwamura, was also a professional baseball player who spent two seasons in the Kintetsu Buffaloes’ farm system. Both were drafted in 1997.

On November 14, information leaked that the highest bidder that won the right to negotiate with Iwamura was the Tampa Bay Devil Rays. Major League Baseball announced the next day that the Rays had made the winning bid; Newsday reported that the Rays had bid $4.5 million. Iwamura was the second player that the Rays had been able to obtain through the posting system; in 2005, they won the rights to negotiate with pitcher Shinji Mori.

He played in the Central League from 1998 to 2006 and won six Golden Glove awards at third base. He also hit 106 home runs in his last three seasons in the Central League, including 32 (with a .311 batting average) in 2006. His best season was 2004, when he hit 44 home runs with 103 RBI and a .300 batting average. In that same season, he also set the Japanese baseball record for strikeouts with 173.

During the 2006 offseason, Iwamura opted to be posted for Major League Baseball. The posts were due to the commissioner of Major League Baseball on November 10, 2006; the highest bidder would be granted the right to negotiate a contract with Iwamura. Incidentally, Daisuke Matsuzaka, who signed a six-year, $52 million contract with the Boston Red Sox that same year, is a close friend of Iwamura.

On December 15, 2006, Iwamura signed a three-year contract with the Rays worth $7.7 million, with a club option for a fourth year at $4.25 million.

On July 10, 2007, Iwamura’s wife Misaki gave birth to their first son, Taiki.

On September 1, 2007, in a game against the New York Yankees, Akinori Iwamura had his bat confiscated in the second inning. The Yankees questioned the flatness of the end of Iwamura’s bat. The bat was sent to the commissioner’s office for inspection, where it was found to be acceptable for use. It was later returned to Iwamura.

Iwamura was designated for assignment by the Pirates on June 16, 2010 after hitting only .182 in Pittsburgh and showing limited range at 2nd base. After failing to work out a suitable trade, Iwamura joined the Pirates’ Triple-A affiliate team Indianapolis Indians on June 22, 2010. He was designated for assignment on September 6 and released on September 8.

On September 13, 2010, Iwamura signed with the Oakland Athletics.

On October 4, 2010, Iwamura was released by the Oakland Athletics.

Iwamura signed with the Tohoku Rakuten Golden Eagles on November 16, 2010. He was expected to bat in the cleanup position for his new team; however, he experienced injuries and had a bad slump during his 2 years with the Golden Eagles. He was released from the Eagles during the 2012 off-season.

Iwamura signed with the Tokyo Yakult Swallows on December 10, 2012.