Van Riemsdyk began his NHL career with the Philadelphia Flyers, the organization that originally selected him second overall at the 2007 NHL Entry Draft, before being traded to the Maple Leafs in 2012. Additionally, he has represented the United States in numerous tournaments.
His younger brother, Trevor, is also a professional hockey player in the NHL, currently with the Carolina Hurricanes. His youngest brother, Brendan, has followed in his brothers' footsteps and is playing for the University of New Hampshire.
Van Riemsdyk attended Christian Brothers Academy in Lincroft, New Jersey, before moving to Ann Arbor, Michigan for his junior year to play for the USA Hockey's National Team Development Program (NTDP). Prior to that, he played from Mite to his first year of Midgets for the Brick Hockey Club in Brick. Although van Riesmdyk was born and raised in New Jersey, he was an avid fan of the rival New York Rangers.
Van Riemsdyk was drafted second overall by the Philadelphia Flyers in the 2007 NHL Entry Draft, following fellow American Patrick Kane, marking the first time in NHL history that Americans were chosen first and second overall in the same draft. Following his draft, van Riemsdyk played the 2007–08 season at the collegiate level in the NCAA with the University of New Hampshire, where he scored 74 points in two seasons under Head Coach Dick Umile.
On April 2, 2009, the Philadelphia Flyers announced that van Riemsdyk would forgo his final two years of college eligibility at the University of New Hampshire to sign an entry-level contract for the 2009–10 season; he had previously spent the remainder of the 2008–09 season playing for the Philadelphia Phantoms, the Flyers' American Hockey League (AHL) affiliate, on an amateur tryout contract.
It was during the 2009–10 pre-season that van Riemsdyk began making an impact for the Flyers. Initially expected to spend his first year of professional hockey in the AHL or, at best, earn a spot midway through the season, van Riemsdyk seized the attention in the Flyers' annual rookie game against the Washington Capitals with four goals and an assist in the team's 7–3 rout of the Capitals' rookies. As he moved into the pre-season, van Riemsdyk established himself as one of the most consistent members of the Flyers outfit, once again seizing attention with two goals against the Detroit Red Wings in a 5–2 Flyers victory.
Van Riemsdyk was included in the Flyers' starting 2009–10 roster to play on a third line with prolific sophomore Claude Giroux. Van Riemsdyk scored his first career NHL point in his NHL debut, recording an assist on a powerplay goal by Mike Richards in a 2–0 victory over the Carolina Hurricanes. His next game saw a return to his home state of New Jersey, where he contributed two more assists against the New Jersey Devils. He suffered a minor concussion shortly after that due to a hit by Washington defenseman Milan Jurčina in the Flyers' home opener and subsequently missed two games. Van Riemsdyk then scored his first goal on October 24 against Tomáš Vokoun of the Florida Panthers.
Van Riemsdyk had an impressive month of November for a rookie, scoring four goals—three of which were game-winners—and five assists. For his performances, he was named the NHL's Rookie of the Month for November.
During the 2010 Eastern Conference Semifinals, Philadelphia (who finished in seventh place in the Eastern Conference during the regular season) found themselves down three games to none against the sixth-seeded Boston Bruins. The Flyers mounted a comeback, however, defeating the Bruins in Game 7, marking just the third time in NHL history a team has won a series after being down by three games. With 2:50 left in the first period, van Riemsdyk scored his first career playoff goal, shifting the game's momentum to the Flyers. Philadelphia went on to win the game 4–3 and the playoff series, four games to three. Not since the New York Islanders came back from a 3–0 deficit against the Pittsburgh Penguins in 1975 had a team won four games in a row to win an NHL playoff series. For van Riemsdyk's momentum-changing goal, he was named the impact player of the game. The Flyers went on to defeat the Montreal Canadiens in the eastern conference finals in 5 games. Van Riemsdyk and The flyers went went on to play in the 2010 Stanley Cup Finals against the Chicago Blackhawks which they lost to in six games in his first finals appearance.
On February 15, 2011, in a game against the Tampa Bay Lightning, van Riemsdyk recorded his first Gordie Howe hat trick with a goal, an assist and a fight in the Flyers' first victory over the Lightning in the season series. On March 26, in a game against the New York Islanders, van Riemsdyk recorded his first career NHL hat-trick in the Flyers' 4–1 victory in front of some of his aunts, uncles and his mother. Two of the three goals were scored in even-strength play, while the third came on the power play.
In the opening round of the 2011 playoffs, van Riemsdyk had a very strong series and was a key to the Flyers' 4–3 series win over the Buffalo Sabres—he tallied four goals in the series, including the eventual game-winner in Game 7 to help the Flyers advance to the next round. He then followed the Buffalo series with three goals in four games against Boston as the Flyers were ultimately swept in four-straight by the Bruins. Despite only playing in 11 games, van Riemsdyk finished third in the entire post-season in total shots with 70—behind only Daniel Sedin and Ryan Kesler of the Vancouver Canucks—and led the post-season in shots per game.
In the subsequent off-season, the Flyers re-signed forward James van Riemsdyk to a six-year, $25.5 million contract extension. Van Riemsdyk, who was entering his third season in Philadelphia, provided the length of the extension on his Twitter. He was scheduled to become a restricted free agent at the end of the 2011–12 campaign, a season in which he finished with then-career highs in points (40), goals (21) and plus-minus rating (+15).
In the 2012 season, van Riemsdyk broke his foot, requiring surgery.
On June 23, 2012, van Riemsdyk was traded to the Toronto Maple Leafs in exchange for defenseman Luke Schenn. During the lockout-shortened 2012–13 with Toronto, van Riemsdyk scored 18 goals and 32 points while playing in all 48 regular season games. The Maple Leafs would make the playoffs for the first time since the 2003–04 season, but fell in the first round to the eventual Stanley Cup finalist Boston Bruins in seven games. Van Riemsdyk finished the playoffs with a team-leading seven points in seven games.
On March 28, 2014, van Riemsdyk tied an NHL record for fastest goal to start off a period, scoring four seconds into the frame. He accomplished the feat against his former team, the Philadelphia Flyers. He currently shares the record with both Claude Provost and Denis Savard. By season's end, he set career-highs in goals (30) and points (61).
Despite a disappointing 2014–15 season for the Maple Leafs, van Riemsdyk led the team in goals (27) while tallying 56 points in all 82 regular season games played. On November 4, 2014, van Riemsdyk recorded an assist on a goal by Cody Franson for his 200th career point.
During the 2015-16 season with the Maple Leafs, van Riemsdyk fractured his left foot on January 11. Up to that point he was leading the team in points (29), and was second in goals (14), and shots (129) after 40 games. It was announced on February 25, 2016 that he was going to be out for the remainder of the season. On October 14, 2016, van Riemsdyk was announced as one of the one hundred greatest Toronto Maple Leafs of all time, placing 100 on the list. On October 15, 2016, the Maple Leafs retired Borje Salming's No. 21, causing van Riemsdyk to change his number from 21 to 25. During the 2016-17 NHL season he finished with 29 goals and career high 33 assists and career high in 62 points. The leafs made it to the 2017 Stanley Cup Playoffs, in which they lost in the first round in six games to the Washington Capitals. Van Riemsdyk recorded 2 goals and 1 assists for a total of 3 points.
Van Riemsdyk's first international experience came when he played in the 2007 World Under-18 Championships (U18), in which the United States won the gold medal. In 2006–07, he scored 33 goals and 30 assists for the U.S. Under-18 National Team, and represented the United States at the 2007 World Junior Championships (WJC) in Sweden. He followed up his inaugural WJC by winning the scoring title at the 2007 U18 with five goals and seven assists. He was named to the tournament all-star team, named the top forward and garnered MVP honors as the United States won silver, only losing against the Russians in the final.
Van Riemsdyk had a great showing at the 2008 WJC, where he led the entire tournament in scoring with five goals and six assists. Despite his efforts, however, the U.S. did not medal, finishing fourth. Nonetheless, he was named to the WJC All-Star Team in 2008. He continued his strong international play with six goals and four assists at the 2009 WJC as the Americans finished in fifth.
Van Riemsdyk made his debut in senior international competition when he played for the United States at the 2011 IIHF World Championship in Slovakia, where Team USA finished eighth. Van Riemsdyk scored one goal in the two games he played.
On January 1, 2014, after van Riemsdyk had played in the 2014 NHL Winter Classic for Toronto against the Detroit Red Wings, he was announced as a member of the U.S. Olympic team for participation at the Winter Olympics in Sochi. The Americans, however, finished a disappointing fourth at the tournament, though in the six games van Riemsdyk played in, he had one goal and a tournament-leading six assists, also leading the tournament in plus-minus.
Van Riemsdyk has two younger brothers who are also hockey players, Trevor, who is a defenseman for the Carolina Hurricanes; and Brendan, who plays for the University of New Hampshire.
In 2012, van Riemsdyk appeared in the movie This is 40 alongside Scott Hartnell, Ian Laperrière and Matt Carle.
On November 24, 2012, van Riemsdyk participated in Operation Hat Trick, a charity hockey game held at Boardwalk Hall in Atlantic City, New Jersey, to raise money for Hurricane Sandy victims.