Mendy's costly mistake was the catalyst for Chelsea's shocking implosion at Elland Road as Brenden Aaronson netted after catching the goalkeeper taking too long in possession.
Rodrigo's fourth goal in three games this season doubled Leeds' advantage before half-time. Jack Harrison completed Leeds' first win over Chelsea since 2002 and their biggest against their old rivals since 1995.
Adding to Chelsea's misery, their Senegal defender Kalidou Koulibaly was sent off for a second booking after tripping Joe Gelhardt in the 84th minute.
Unbeaten after three games, Leeds sit second in the table as boss Jesse Marsch builds on the momentum established during the club's successful fight to avoid relegation last season.
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