- Bats Right, Throws Right
- Height 5' 7", Weight 160 lbs.
- High School Kouryo High School
- School Meiji University
Iwamoto was a pitcher at the 1931 Summer Koshien tournament, where he led Kouryo into the quarterfinals before losing to eventual champion Chukyo. After graduating from Meiji in 1934, he joined the corporate team Tokyo Club, which won the 1935 Intercity Baseball Tournament. Also on the team was future Hanshin Tigers and Korakuen Eagles manager Shigeo Mori. After the establishment of the Japanese Professional Baseball League in 1936, the corporate team's days were numbered and it disbanded in 1937.
Iwamoto then joined the pro league's newest team, Nankai, in 1938 and wore number 30. He didn't get into any games that year and was called up to serve in the military in 1939 and was not on the team's roster in that year. Iwamoto returned to Nankai in 1940 and would play with the club for the next three years hitting 7 home runs in 1941 and 1942 in pitcher-dominated seasons. Iwamoto managed Nankai for 23 games in 1942 in the interim between the firing of Hachiro Mitani and the hiring of Kisaku Kato. He was called up again to the military after the 1942 season. After the war Iwamoto played with All-Hiroshima and the Beppu-based Uera team where he served as player-manager.
Iwamoto returned to the JBL with the Taiyo Robins in 1949 where he hit 8 home runs in 221 AB in 1949. He followed that up by hitting 39 home runs in 1950, and 31 home runs in 1951. Iwamoto hit the first Central League home run in 1950. He also stole 34 bases in 42 attempts in 1950, to go with a .319 batting average becoming Japan's first 30-30 player with Kaoru Betto. As an outfielder he started eight double plays in 1951, a record. On August 1, 1951 he hit four home runs in a game (a record)and added a double setting a record for total bases with 18. Despite his home run hitting feats, Iwamoto never won a home run title, as he trailed Makoto Kozuru and Michio Nishizawa in 1950, and trailed Noboru Aota in 1951. Iwamoto was selected to three all-star teams (1951, 1952, 1953) and he was elected for Best Nine in 1950 and 1951. In 1952, he was hit by 24 pitches, a NPB record for 55 years until Greg LaRocca broke it. He retired as a full-time player in 1953 at the age of 41.
Iwamoto became the manager of the Toei Flyers in 1956; he had previously served player-coach for the Shochiku Robins in 1951. As a player-manager he got into 101 games in 1956 and 1957. He become the oldest player to hit a home run in NPB history, when he hit one at the age of 45 years, 5 months and 7 days on August 18, 1957. After being let go by the Flyers in after the 1960 season, he stayed on for a year as a coach and then moved onto the Kintetsu Buffaloes where he coached from 1962 to 1964. He was the team's manager from 1965 to 1966. He entered the Japanese Baseball Hall of Fame in 1981.
His career numbers include 123 home runs, a .275/.361/.446 line and a 370 - 532 - 20 record as a manager.