[5] He made his club debut in the 1996 FA Charity Shield against Newcastle United, appearing as a substitute for Nicky Butt in a 4–0 win.

[5] in December 1997, Poborský moved to the Portuguese side Benfica for an undisclosed fee, signing a three-and-a-half-year contract to play for the club, which was managed by Graeme Souness.

[3] In July 2002, Poborský returned to the Czech Republic after signing as a free agent with Sparta Prague, where he became the highest-paid footballer playing in the country.

[15] Karel Poborský's first international appearance, which was against Turkey on 23 February 1994,[16] was also the first match for the Czech Republic's national side after the dissolution of Czechoslovakia.

After losing against Germany in the opening group match, a Poborský cross resulted in a goal for teammate Pavel Nedvěd in a 2–1 win against Italy.

In the knockout stages of the tournament, Poborský scored his first international goal; he lobbed the ball over Portugal's advancing goalkeeper Vítor Baía, settling the game, which the Czech Republic won 1–0.

[20] In Czechia's semi-final against France, Poborský was among players who scored penalties in the shootout after a goaless match, which the Czech Republic won.

He scored his country's first goal of the tournament from the penalty spot, against France in a 2–1 group-stage loss that resulted in Czechia's elimination from the championship.

[23] In the nation's final match of the group stage, against Denmark, Poborský provided the pass from which Vladimír Šmicer opened the scoring, with the game finishing 2–0.

[26] In Czechia's second group-stage match against the Netherlands, the Dutch side took a 2–0 lead but Poborský assisted the game-winning third Czech goal, which Vladimír Šmicer scored.

Poborský and eight other players were rested for the final group-stage match against Germany, in which he played from the 70th minute as a substitute for Jaroslav Plašil.

Following the 2006 World Cup, he announced his retirement from international football, having scored eight goals and made 118 appearances for his national team, a record at the time.

[34] In 2016, Poborský was put into a medically induced coma after contracting a brain infection that paralysed the muscles in his face and caused hypersensitivity to light.