Higashio was a star in the Koshien high school baseball tournament, helping his team advance to the semi-finals in the spring of 1968. He was drafted in the first round by the Nishitetsu Lions (the current Saitama Seibu Lions) later that year. He lost confidence in his pitching ability after seeing the high level of pitching in the Pacific League, and requested the team to convert him to a position player. The team accepted his request, but quickly withdrew it when the Black Mist Scandal erupted in the 1969 off-season. The ace of the Lions pitching staff, Masaaki Ikenaga, was banished from the professional leagues because of the scandal, and Higashio was forced into pitching a full year in the Lions starting rotation. His inexperience left him with a torrid 5.15 ERA in 40 games that year. Higashio made improvements in the following years, but still led the league in losses from 1971–1972. He pitched over 300 innings in 1972, but led the league in losses, hits given up, home runs given up, and runs given up. He also allowed over 100 walks in each of his seasons from 1971–1973.
The Lions team was sold by the Nishi-Nippon Railroad in 1973, becoming the Taiheiyo Club Lions, and was sold again to become the Crown Lighter Lions in 1978. It was during this turbulent period that Higashio emerged as the ace of the Lions pitching staff. He marked a 2.38 ERA in 1975, and led the league with 23 wins (but also in losses). He won 23 games again in 1978, pitching over 300 innings for the third time in his career. The Lions finally obtained financial stability in 1979, becoming the Seibu Lions, and won the Japan Series in 1982 and 1983. Higashio marked the lowest ERA in the league (2.92), and led the league in wins to receive the MVP award in 1983. The Lions won the Pacific League championship four years in a row from 1985–1988 (including three more Japan Series wins), and Higashio won his second MVP award in 1987. He announced his retirement in 1988, and is remembered as one of the leading Japanese pitchers in the 1980s.