Taegeuk Warriors skipper can't prevent the seven-time French champions from the drop, while Park Ji-Sung suffers another Uefa Champions League defeat
Heading into the contest at Stade Louis II, Monaco knew a victory would ensure their survival despite the fact they were sitting in the bottom three, as sides above them were playing each other.
Park combined well with strike partner Georgie Welcome who fired an early volley off target, but it was the visitors who looked more likely with Jimmy Briand going close on a few occasions.
Monaco's fate was settled on 67 minutes, when Pape Diakhite scored from close-range, and 15 minutes later Lisandro Lopez landed the killer blow to double Lyon's advantage and send the 2000 French champions down to Ligue 2 for the first time since 1976.
Park, who was replaced on 76 minutes by Daniel Niculae, played reasonably well and was regularly available as an outlet, but often found himself isolated.
The 25-year-old Taegeuk Warriors skipper must now weigh up whether to stay with Monaco, who he has been with since 2008, in the second tier of French football.
In the Uefa Champions League final, Park Ji-Sung was on the losing side for the second time in three years as Manchester United were beaten 3-1 by Barcelona.
The Korean midfielder, who missed out on playing in the Red Devils' 2008 European success, was energetic early in Saturday's clash at Wembley Stadium and industrious throughout, but his influence wore off.
United boss Sir Alex Ferguson might have hoped Park could destroy Barcelona's creativity in midfield but he found the going tough against his opponents and wasn't terribly effective.
Jung Jo-Gook (AJ Auxerre): Came off the bench as a 82nd-minute substitute in 2-1 win at Lorient which moved Auxerre up to a top-half Ligue 1 finish.
Nam Tae-Hee (Valenciennes): Was an unused substitute in a 2-1 victory over Nice which sealed their survival and saw them finish the campaign in 13th.