As a youth, Meszároš played in the 1999 Quebec International Pee-Wee Hockey Tournament with a team from Bratislava.
He later played for Dukla Trenčín in the Slovak Extraliga as a 17-year-old in the 2002–03 season.
After two seasons with Trenčín, he was drafted 23rd overall in the 2004 NHL Entry Draft by the Ottawa Senators.

The following season, he went overseas to North America to play junior ice hockey in the Western Hockey League (WHL) with the Vancouver Giants.
Meszároš recorded 41 points in 59 games with Vancouver — fifth in WHL rookie scoring — and was named to the Western Conference's Second All-Star Team.
He was also named the Giants' Rookie of the Year and Top Defenceman.
Nearly six years later, he was inducted into the Giants' Ring of Honour, a series of banners inside the Pacific Coliseum commemorating the team's best alumni.

In the 2005–06 season, Meszároš made the jump to the NHL with the Senators.
Meszároš registered his first NHL point, an assist, on 10 October 2005, in a 6–5 shootout win over the Toronto Maple Leafs.
Two months later, he scored his first NHL goal on 9 December — a game-tying goal in the third period of a shootout loss against the Vancouver Canucks.
He finished his rookie season with 10 goals and 39 points, and was named to the NHL All-Rookie Team with the Calgary Flames' Dion Phaneuf.
Playing on a defensive pairing with Zdeno Chára, Meszároš' plus-minus rating of +39 was tied for third in the league overall and first among rookies.

The following year, Meszároš registered seven goals and 35 points and added seven points in the playoffs as the Senators made their first franchise appearance in the Stanley Cup Finals against the Anaheim Ducks.
The Senators, however, were defeated by Anaheim in five games.
Meszároš finished second on the team in playoff plus-minus, behind Wade Redden, at +5.
His sophomore season also included an appearance in the 2007 YoungStars Game with teammate Patrick Eaves, in which he scored two goals in a 9–8 victory over the Western Conference.

Although Meszároš produced at a similar pace in 2007–08, the Senators finished a disappointing seventh in the Eastern Conference and were swept by the Pittsburgh Penguins in the first round of the playoffs.
Meszároš became a restricted free agent in the off-season and, unable to come to terms, the Senators traded him to the Tampa Bay Lightning on 29 August 2008 in exchange for Filip Kuba, Alexandre Picard and the San Jose Sharks' first-round draft pick (previously acquired) in 2009.
The following day, he agreed to a six-year, $24 million contract.

On 1 July 2010, the first day of the NHL's free agency period, Meszároš was traded to the Philadelphia Flyers in exchange for a second round pick in 2012.

In his first season with the Flyers, Meszároš won the Barry Ashbee Trophy, awarded to the best defenceman on the team.

On 5 March 2014, the Flyers traded Meszároš to the Boston Bruins in exchange for a conditional 2014 third-round draft pick.
He scored a power play goal in his debut as a Bruin on 9 March en route to a 5–2 road win against the Florida Panthers.

On 1 July 2014, Meszároš signed a one-year, $4 million contract with the Buffalo Sabres.
In the 2014–15 season, expected to contribute on the rebuilding Sabres blueline, Meszároš struggled to play consistently to match his contract, appearing in 60 games with only 14 points.

As a free agent from the Sabres in the off-season, Meszároš went un-signed and later accepted a professional try-out contract to attend the Colorado Avalanche training camp on 15 September 2015.
After suffering an early injury during training camp, he was unable to make an impression and was later released on 30 September.
On 30 October 2015, Meszaros signed with Sibir Novosibirsk of the Kontinental Hockey League (KHL).
As a youth, Meszároš played in the 1999 Quebec International Pee-Wee Hockey Tournament with a team from Bratislava. He later played for Dukla Trenčín in the Slovak Extraliga as a 17-year-old in the 2002–03 season. After two seasons with Trenčín, he was drafted 23rd overall in the 2004 NHL Entry Draft by the Ottawa Senators.
The following season, he went overseas to North America to play junior ice hockey in the Western Hockey League (WHL) with the Vancouver Giants. Meszároš recorded 41 points in 59 games with Vancouver — fifth in WHL rookie scoring — and was named to the Western Conference's Second All-Star Team. He was also named the Giants' Rookie of the Year and Top Defenceman. Nearly six years later, he was inducted into the Giants' Ring of Honour, a series of banners inside the Pacific Coliseum commemorating the team's best alumni.
In the 2005–06 season, Meszároš made the jump to the NHL with the Senators. Meszároš registered his first NHL point, an assist, on 10 October 2005, in a 6–5 shootout win over the Toronto Maple Leafs. Two months later, he scored his first NHL goal on 9 December — a game-tying goal in the third period of a shootout loss against the Vancouver Canucks. He finished his rookie season with 10 goals and 39 points, and was named to the NHL All-Rookie Team with the Calgary Flames' Dion Phaneuf. Playing on a defensive pairing with Zdeno Chára, Meszároš' plus-minus rating of +39 was tied for third in the league overall and first among rookies.
The following year, Meszároš registered seven goals and 35 points and added seven points in the playoffs as the Senators made their first franchise appearance in the Stanley Cup Finals against the Anaheim Ducks. The Senators, however, were defeated by Anaheim in five games. Meszároš finished second on the team in playoff plus-minus, behind Wade Redden, at +5. His sophomore season also included an appearance in the 2007 YoungStars Game with teammate Patrick Eaves, in which he scored two goals in a 9–8 victory over the Western Conference.
Although Meszároš produced at a similar pace in 2007–08, the Senators finished a disappointing seventh in the Eastern Conference and were swept by the Pittsburgh Penguins in the first round of the playoffs. Meszároš became a restricted free agent in the off-season and, unable to come to terms, the Senators traded him to the Tampa Bay Lightning on 29 August 2008 in exchange for Filip Kuba, Alexandre Picard and the San Jose Sharks' first-round draft pick (previously acquired) in 2009. The following day, he agreed to a six-year, $24 million contract.
On 1 July 2010, the first day of the NHL's free agency period, Meszároš was traded to the Philadelphia Flyers in exchange for a second round pick in 2012.
In his first season with the Flyers, Meszároš won the Barry Ashbee Trophy, awarded to the best defenceman on the team.
On 5 March 2014, the Flyers traded Meszároš to the Boston Bruins in exchange for a conditional 2014 third-round draft pick. He scored a power play goal in his debut as a Bruin on 9 March en route to a 5–2 road win against the Florida Panthers.
On 1 July 2014, Meszároš signed a one-year, $4 million contract with the Buffalo Sabres. In the 2014–15 season, expected to contribute on the rebuilding Sabres blueline, Meszároš struggled to play consistently to match his contract, appearing in 60 games with only 14 points.
As a free agent from the Sabres in the off-season, Meszároš went un-signed and later accepted a professional try-out contract to attend the Colorado Avalanche training camp on 15 September 2015. After suffering an early injury during training camp, he was unable to make an impression and was later released on 30 September. On 30 October 2015, Meszaros signed with Sibir Novosibirsk of the Kontinental Hockey League (KHL).
Andrej Meszároš