After high school he attended Vincennes University, a junior college in Indiana, before transferring to the University of Nevada-Las Vegas, where he played collegiately.
Nicknamed The Matrix (coined by TNT analyst Kenny Smith) during the preseason of his rookie season, Marion was selected to the US Olympic men's basketball team in 2004. In 2005, he was named a reserve on the Western Conference All-Star Team and selected to the 2004-05 All-NBA Third Team. That year he became the first player since David Robinson in 1992 to average in the top five in rebounding and steals since the league started to track steals in 1973. Marion did it again in 2005-06. During the 2005 All-Star Weekend, Marion teamed up with WNBA Rookie of the Year Diana Taurasi of the Phoenix Mercury, and Suns legend Dan Majerle in the RadioShack Shooting Stars to run away with the title. Marion was also selected as a Western Conference reserve on the 2006 All-Star team.
The 2005-06 NBA season was perhaps the best season of his career. He was the only player in the NBA ranked in the top 20 in points, rebounds, steals, blocks, field goal percentage and minutes. He finished the season leading the Suns in points per game (21.8), rebounds per game (11.8), blocks per game (1.7), and steals per game (2.0). Marion also ranked 3rd in efficiency. He helped fill in the void left by star Amare Stoudemire, who did not play virtually the entire season due to injury. Marion also earned a spot in the NBA All-Star Game for the third time.
On March 5, 2006, Marion was one of 23 players named to the USA men's senior national team that finished third in the 2006 FIBA World Championship. Marion was forced to withdraw from the team before the tournament began due to a knee injury, but he will still be among the players considered to represent the U.S. in the 2007 Tournament of the Americas in Las Vegas and the 2008 Olympics in Beijing. Marion played for the senior United States National Basketball Team in the 2002 FIBA World Championship and the 2004 Athens Olympics. He has earned 22 total international caps for the USA.
On September 26, 2007, following a summer filled with rumors and trade speculation involving possible deals with the Boston Celtics, Los Angeles Lakers, and Utah Jazz, Marion requested to be traded. Citing the rumors and a refusal on the part of Phoenix Suns management to negotiate for a contract extension, Marion called his relationship with the Suns a "bad marriage", and stated that it was time for him to leave Phoenix. By the start of the season, though, Marion continued to play well for the Suns.
During the 2006-2007 season, Marion was one of two players (the other Kevin Garnett) to rank in the top 40 in points per game, rebounds per game, field goal percentage, blocks per game, steals per game, and minutes per game.
On February 6 2008, the Phoenix Suns traded Shawn Marion and teammate, Marcus Banks, to the Miami Heat for center Shaquille O'Neal which ended the 9 year run with the Phoenix Suns. Marion and Dwyane Wade were unable to jell in the second half of the season, with both missing time with injuries. Marion had an opt-out clause in his contract, but did not exercise it. He had been negotiating for an extension, but an agreement could not be reached. He will be a free agent in the summer of 2009.
|- | align="left" | 1999–00 | align="left" | Phoenix | 51 || 38 || 24.7 || .471 || .182 || .847 || 6.5 || 1.4 || .8 || 1.0 || 10.2 |- | align="left" | 2000–01 | align="left" | Phoenix | 79 || 79 || 36.2 || .480 || .256 || .810 || 10.7 || 2.0 || 1.7 || 1.4 || 17.3 |- | align="left" | 2001–02 | align="left" | Phoenix | 81 || 81 || 38.4 || .469 || .393 || .845 || 9.9 || 2.0 || 1.8 || 1.1 || 19.1 |- | align="left" | 2002–03 | align="left" | Phoenix | 81 || 81 || 41.6 || .452 || .387 || .851 || 9.5 || 2.4 || 2.3 || 1.2 || 21.2 |- | align="left" | 2003–04 | align="left" | Phoenix | 79 || 79 || 40.7 || .440 || .340 || .851 || 9.3 || 2.7 || 2.1 || 1.3 || 19.0 |- | align="left" | 2004–05 | align="left" | Phoenix | 81 || 81 || 38.8 || .476 || .334 || .833 || 11.3 || 1.9 || 2.0 || 1.5 || 19.4 |- | align="left" | 2005–06 | align="left" | Phoenix | 81 || 81 || 40.3 || .525 || .331 || .809 || 11.8 || 1.8 || 2.0 || 1.7 || 21.8 |- | align="left" | 2006–07 | align="left" | Phoenix | 80 || 80 || 37.6 || .524 || .317 || .810 || 9.8 || 1.7 || 2.0 || 1.5 || 17.5 |- | align="left" | 2007–08 | align="left" | Phoenix | 47 || 47 || 36.4 || .526 || .347 || .713 || 9.9 || 2.1 || 2.0 || 1.5 || 15.8 |- | align="left" | 2007–08 | align="left" | Miami | 16 || 15 || 37.6 || .459 || .258 || .690 || 11.2 || 2.5 || 1.9 || .9 || 14.3 |- | align="left" | Career | align="left" | | 676 || 662 || 37.8 || .481 || .341 || .821 || 10.1 || 2.0 || 1.9 || 1.3 || 18.3 |- | align="left" | All-Star | align="left" | | 4 || 0 || 19.5 || .575 || .000 || .500 || 6.5 || 3.0 || 1.5 || .5 || 12.5 |}
|- | align="left" | 1999–00 | align="left" | Phoenix | 9 || 9 || 31.2 || .419 || .167 || .818 || 8.8 || .8 || .7 || 1.6 || 9.1 |- | align="left" | 2000–01 | align="left" | Phoenix | 4 || 4 || 34.8 || .371 || 1.000 || .857 || 8.3 || .8 || 1.5 || 1.5 || 14.8 |- | align="left" | 2002–03 | align="left" | Phoenix | 6 || 6 || 47.0 || .374 || .321 || .846 || 11.7 || 2.0 || 1.8 || 1.8 || 18.5 |- | align="left" | 2004–05 | align="left" | Phoenix | 15 || 15 || 42.3 || .484 || .419 || .769 || 11.8 || 1.5 || 1.4 || 1.7 || 17.6 |- | align="left" | 2005–06 | align="left" | Phoenix | 20 || 20 || 42.5 || .489 || .314 || .881 || 11.7 || 1.6 || 1.9 || 1.1 || 20.4 |- | align="left" | 2006–07 | align="left" | Phoenix | 11 || 11 || 41.4 || .500 || .353 || .667 || 10.4 || 1.2 || 1.5 || 1.7 || 16.9 |- | align="left" | Career | align="left" | | 65 || 65 || 40.6 || .463 || .346 || .811 || 10.9 || 1.4 || 1.5 || 1.5 || 17.1 |}