Initially Jean Cornelis played as a striker for his first club FC. Lot. Then he went to Racing Tournai as a left back. He became famous in Anderlecht team of the 60s, playing next to Jurion, Puis and Van Himst. He played 19 games for the national team. His first one was against Spain in 1962, his best one on Wembley in 1964 where he scored against England (2-2). We also remember him for his red card against Real Madrid. After the game he openly criticized the referee (the case Barberan).

During his last two seasons for Sporting Anderlecht, he had sharp competition of Maurice Martens and Jos Volders.

His last game for Belgium was against West-Germany in '68. He left Anderlecht in 1971 and went to Beveren. But that became a bad decision. Beveren relegated to the second division. Cornelis left the club for Crossing, where Vandenbergh, Claessen and Masopust played. But again it went wrong. Also Crossing relegated and Jean Cornelis ended his carreer. He became trainer for La Louvière, Union and again La Louvière before finally returning to Sporting Anderlecht to train the youth teams.

Cornelis remained active for his club until his very last days. In March 2016 he suffered a heart attack while attending Anderlecht's women team. He was rushed to the hospital in a coma from which he never woke up again. Cornelis died on March 21 of the year 2016. He became 74 years old.