[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.

[27] At the quarter-final stage, Poborský provided two more assists as the Czech side defeated Denmark 3–0, with all of the goals coming in the second half.

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.