Toshikazu Yamanishi has been participating in international competitions since 2013. He qualified for the U18 World Championships in Donetsk after setting his best time of 41:13 minutes over the 10-kilometer distance in the spring. There he won the gold medal in the 10-kilometer walk in 41: 53.80 minutes. As a result, he mainly competed in walking competitions in his Japanese homeland, both in national and university championships.
In 2017 he competed at the Universiade in Taipei over 20 kilometers, which he was able to win in 1: 27.30 hours. A year later he started at the Asian Games in Jakarta over 20 kilometers. There he won the silver medal. He improved his time by more than 5 minutes compared to the Universiade.
For him, 2019 is the most successful year in sport so far. In the spring he set his best time of 1:17:15 h and remained just under a minute above the world record time of his compatriot Yūsuke Suzuki . This made him the fastest of the year to travel to the World Championships in Doha in October . There he won the world title in 1:26:34 h. Due to the high temperatures and high humidity, it was the slowest world championship race to date. With his victory, the Japanese emerged as winners from both men's walking competitions.