Iwakuma, 30, answered reporters' questions Tuesday in a written response through interpreter Antony Suzuki before he heads back to Japan.
"First and foremost, I am very happy with how the Seattle Mariners treated me throughout the season and for their evaluation in me as a starter," Iwakuma said. "We have great teammates here along with an outstanding coaching staff. My family was a big factor in our decision [and] we are very happy and satisfied with Seattle and we look forward to staying for at least two years."
Iwakuma signed a two-year deal that will pay him $6.5 million each of the next two seasons, with a $1 million buyout on a $7 million option for 2015 as well.
Iwakuma was 9-5 with 3.16 ERA over 30 appearances in his first season in the Majors. He began the year as a long reliever, then transitioned into the rotation midseason and went 8-4 with a 2.65 ERA in 16 starts.
"There was a lot I learned this year as a reliever in the beginning, just a whole new experience in a different role," he said. "I was very happy given the opportunity to become a starter again and I'm glad I was able to show what I can do. As I became comfortable in my role, I learned a lot about the hitters here and how they are different from Japan."
It's not known yet whether Iwakuma will pitch for Japan in the World Baseball Classic next spring. He was a member of Japan's Classic championship team in 2009. But he did say he plans to get more prepared in the offseason this year after his slow start last spring with the Mariners.
"I do not see myself changing anything dramatically," he said. "I would like to prepare for the season earlier than the past and come to Spring Training ready to go as opposed to getting prepared in Spring Training."
Iwakuma knows the Mariners are moving the fences in next season at Safeco Field but said he wasn't worried about it. He was 6-3 with a 2.49 ERA in 17 games at Safeco last year, while going 3-2 with a 4.20 ERA in 13 road outings.
"Obviously, we know that the ball doesn't travel well in Seattle," Iwakuma said. "I just need to continue locating my pitches and keeping the ball down in the zone."
The signing of Iwakuma gives the Mariners three veteran starters atop their rotation with Felix Hernandez, Jason Vargas and Iwakuma. Young returners Blake Beavan, Erasmo Ramirez and Hector Noesi figure to compete for the final two starting spots with top prospects Danny Hultzen, James Paxton, Taijuan Walker and Brandon Mauer.