Mononobe competed as a member of the Japanese team in the 4 × 200 m freestyle relay at the 2008 Summer Olympics in Beijing. Despite missing out the individual spot in the 200 m freestyle, he managed to place third at the Olympic trials in Tokyo (1:49.04) to earn a selection on the relay team. Teaming with Hisato Matsumoto, Yoshihiro Okumura, and Sho Uchida in the final, Matsumoto swam the anchor leg to close the race with a split of 1:48.62, but the Japanese team had to settle for seventh place with 7:10.31.