The win also confirmed Jeonbuk's place in next year's Asian Champions League.
Jeonbuk outclassed Cerezo Osaka in the second leg of their Champions League quarter-final in midweek and carried on where they left off against Sangju.
Lee, who scored four times on Tuesday, put the hosts ahead just before the half-hour and Lee Sung-hyun made it two on the stroke of half-time. Lee Sung-jae pulled a goal back for the visitors after 57 minutes but Lee Sung-hyun restored Jeonbuk's two-goal advantage on the hour.
Eninho netted the fourth goal midway through the second period and Lee Dong-gook grabbed his second two minutes before the end.
"The players today [Monday] showed their desire to finish top," said Jeonbuk coach Choi Kang-hee.
"It doesn't matter what the opposition, they kept going and kept looking to score goals. We hope to win the league at Suwon next time around."
The other game of the day was between two teams that were also engaged in continental competition in midweek.
Suwon, who also booked a place in the last four with a win over Iranians Zob Ahan, defeated FC Seoul - who were eliminated by Al Ittihad - 1-0 to jump above the capital club into third.
A full house of almost 45,000 at Suwon World Cup Stadium saw Stevica Ristic grab the decisive goal with 12 minutes remaining.