Nash was born in Johannesburg, South Africa, to a Welsh mother, Jean, and English father, John, on February 7, 1974. His family moved to Regina, Saskatchewan when he was 18 months old, before settling in Victoria, British Columbia. He therefore holds British as well as Canadian citizenship. Before the family settled in Canada, his father played professional soccer in various parts of the world. Nash often played soccer and ice hockey with his younger brother Martin, and did not start playing basketball until he was 12 or 13 years old. In grade eight, however, he told his mother that one day he would play in the NBA and would become a star. He was neighbour to Future NHL stars Russ and Geoff Courtnall, who used to babysit him and played soccer coached by Nash's father.

In the early 1990s, Nash was cut from the Canadian junior national team by head coach Ken Olynyk, father of Future NBA player Kelly Olynyk. At age 17, he was the youngest member of Team Canada at the 1991 World University Games, where the team won a silver medal.

Nash originally attended Mount Douglas Secondary School in Saanich, British Columbia, but after his grades began to drop, his parents decided to enroll him at St. Michaels University School, a private boarding school in Victoria. There, he starred in basketball, soccer, and rugby union. While playing basketball during his senior season, Nash averaged 21.3 points, 11.2 assists, and 9.1 rebounds per game. In the 1991–92 season, he led his team in his final year to the British Columbia AAA provincial championship title, and was named the province's Player of the Year.

Nash was awarded a scholarship by Santa Clara for the 1992–93 season. At that time, it had been five years since the Broncos appeared in the NCAA tournament. That changed when Nash led the Broncos to a WCC Tournament title and an upset win over the No. 2 seeded Arizona in the first round of the NCAA Men's Division I Basketball Tournament. In that game, Nash scored six straight free throws in the last 30 seconds of the contest. Although Santa Clara was defeated by Temple in the next round, the 1992–93 campaign was considered a successful one. However, the Broncos failed to sustain the momentum the following season, and only managed a 5–7 record in the conference. The team rebounded in the 1994–95 season, with Nash being named Conference Player of the Year and the Broncos topping the WCC. Featuring the league leader for scoring and assists in Nash, the Broncos returned to the NCAA tournament, but they were defeated by Mississippi State. After the season, Nash contemplated turning professional, and decided against it when he learned that he would probably not be considered as a first-round pick in the 1995 NBA draft.

In 1993, while in college, he played for the senior national team at the Tournament of the Americas and competed in the Canada Games (for the British Columbia team) and World University Games. He won a bronze medal at the Canada Games and won a silver medal at the World University Games, losing to Team USA in a closely contested final, which included players such as Michael Finley and Damon Stoudamire.

In the 1995–96 season, Nash began attracting the attention of the national media and professional scouts. He had spent the summer before that honing his skills, playing with the Canadian national team and working out with the likes of established NBA players Jason Kidd and Gary Payton. Santa Clara again captured the WCC title, and for the second consecutive year, Nash was named Conference Player of the Year, the first Bronco to do so since Kurt Rambis. He scored 28 points in leading the No. 10 seed Broncos to a first round upset win over No. 7 seed Maryland, but then the Broncos were eliminated by Kansas. Nash's performances ensured that he earned an honourable mention All-America as a senior by The Associated Press and the USBWA. He also finished his career as Santa Clara's all-time leader in career assists (510), free-throw percentage (.862), and made and attempted three-pointers (263–656). He remains third on the school's all-time scoring list (1,689), and holds Santa Clara's single-season free-throw percentage record (.894). In September 2006, Nash had his jersey (#11) retired, becoming the first Santa Clara student-athlete to receive that honour.

After graduating with a degree in sociology, Nash was selected 15th overall by the Phoenix Suns in the first round of the 1996 NBA draft. Upon hearing the draft announcement, Suns fans booed in disapproval of the relatively unknown player. This was because despite his impressive college accomplishments, Nash had not played in one of the major college conferences. A major influence in Phoenix's choice was assistant coach Donnie Nelson, who met Nash back in high school as he was coached by Nelson's friend Ken Shields, and would eventually befriend the player as he played in Santa Clara. During his first two seasons in the NBA, Nash played a supporting role behind NBA star point guards Kevin Johnson, Sam Cassell, and later, Jason Kidd. Both Johnson and Cassell had NBA Finals experience, while Kidd was the second overall pick in the 1994 NBA draft and already an All-Star when he arrived at Phoenix.

It was in Dallas that Nash established himself as a formidable point guard, beginning a decade as one of the game's top players. During his first year as a Maverick (the lockout-shortened 1998–99 season) he started in all 40 games he played in, and averaged 7.9 points, 2.9 rebounds and 5.5 assists per game. The 19–31 Mavericks failed to make the playoffs but in the 1999–2000 season, the team's prospects improved considerably. Nash missed 25 mid-season games due to an ankle injury, but came back to record six double-doubles in the last month of play. He finished the season with averages of 8.6 points and 4.9 assists per game. More importantly for the team, second-year teammate and friend Dirk Nowitzki was blossoming quickly into a top player, veteran Michael Finley was having an All-Star-calibre year, and the team's new owner, Billionaire Mark Cuban, was bringing new Energy and excitement to the franchise. Nash now had a supportive environment in which he could thrive.

At the 1999 Tournament of the Americas, Nash led Canada to the silver medal, qualifying the team for the Olympics for the first time in 12 years; he was named tournament MVP. Nash captained Canada at the 2000 Sydney Olympics. He led Canada to win their round robin group with a victory over Spain and a stunning 83–75 win over favoured Yugoslavia when he scored 26 points with eight rebounds and eight assists. Canada was eliminated in the quarterfinals with a five-point loss to France and Nash left the court in tears. Nash expressed disappointment with the result, saying "It hurts a lot. I feel like I let everybody down. We could have been in the championship game. We were good enough." Nevertheless, he did see a possible silver Li Ning, saying "Hopefully kids [in Canada] will be inspired to play—that's what I really hope." A victory in its final game of the tournament, a placement game against Russia, enabled Canada to salvage seventh place. Nash's Olympic performance propelled him to stardom in Canada and he finished fifth in voting for the 2000 Lionel Conacher Award, which is handed out to the Canadian male athlete of the year.

In the 2000–01 season, Nash averaged 15.6 points and 7.3 assists per game in a breakout season. With Nash directing the team's offense, Nowitzki and Finley playing at their best, and the acquisition of All-Star Juwan Howard complementing the high-scoring trio, the Mavericks earned a playoff berth for the first time in more than a decade. Dallas lost in the Western Conference Semifinals four games to one to the San Antonio Spurs, but it marked the beginning of a memorable run for Nash and the Mavericks. In the 2001–02 season, Nash posted career-highs of 17.9 points and 7.7 assists per game and earned a spot in the NBA All-Star Game and on the All-NBA Third Team. He was now an All-Star, increasingly appearing in television commercials and, with Finley and Nowitzki, a part of the Dallas Mavericks "Big Three." Dallas earned another trip to the playoffs but lost again in the Semifinals to the Sacramento Kings four games to one.

In 2001, Nash formed the Steve Nash Foundation. Through grants to public Service and nonprofit entities, the foundation aims to foster health in kids by funding projects that provide services to children affected by poverty, illness, abuse, or neglect, and create opportunity for education, play, and empowerment. It focuses its resources on communities in Phoenix, Arizona, and British Columbia, Canada. It was given charitable status in 2004. This foundation was awarded the Steve Patterson Award for Excellence in Sports Philanthropy in 2008. Nash also founded the Jim Jennings Memorial Endowment Fund, established in honour of a volunteer staff member at Santa Clara University who served the basketball team for more than 20 years.

Nash closely replicated his previous season's performance in the 2002–03 season, averaging 17.7 points and 7.3 assists per game, again earning All-Star and All-NBA Third Team honours. Nowitzki and Nash led the Mavericks from a 14-game winning streak to open the season all the way to the Western Conference Finals, where they lost to the eventual NBA champions, the San Antonio Spurs four games to two. It was only the second Conference Finals appearance in the franchise's history. The 2003–04 season saw an offensively boosted Mavericks roster (with the acquisitions of Antoine Walker and Antawn Jamison) but a dip in Nash's scoring contributions. As a result, he was not selected for the All-Star and All-NBA team rosters even though he achieved new career highs in assists per game (8.8) and free-throw accuracy (91.6%). In the playoffs, the fifth-seeded Dallas failed to make progress yet again as the Sacramento Kings saw them off four games to one.

Nash is also known for his outspoken political views. He was an early, public opponent of the 2003 Iraq War, wearing a custom-made T-shirt to the 2003 NBA All Star Game that stated "No war – Shoot for peace". Although Nash did get positive support from teammate Nick Van Exel among others, he also drew criticism from David Robinson, a former Naval officer and fellow NBA player as well as commentators like Skip Bayless who criticized Nash as being uninformed and advised him to "just shut up and play". Nash has also been critical of Arizona's SB1070, legislation which seeks to aggressively address illegal immigration, as he felt "the law obviously can target opportunities for racial profiling." In August 2017, Nash was critical of President Donald Trump in the aftermath of the 2017 Unite the Right rally, stating that "to defend white supremacists and then slang his shitty ass grape juice pretty much sums the man up", referring to a winery Donald Trump has in Charlottesville, Virginia.

In 2001, Nash met Alejandra Amarilla in Manhattan. They married in June 2005 and had twin daughters, Lola and Bella, born on October 14, 2004 and a son, Matteo, born November 12, 2010. On the day of his son's birth, Nash made a statement to Life & Style in which he announced the birth but called it a "bittersweet moment", revealing that he and his wife had "lived separately for the past several months" and are "in the process of dissolving" their marriage. In March 2016, Nash became engaged to Lilla Frederick, a former Pepperdine University and junior women's U.S. team volleyball player. They married in September 2016. In July 2017, Frederick gave birth to their son, Luca Sun Nash, at Cedars-Sinai Hospital in Los Angeles, California.

The next season, Stoudemire suffered a serious knee injury, and Johnson and Quentin Richardson were traded away. The Suns were not expected to repeat their successful 2005 season, but with Nash directing the same high-tempo offence, the team compiled a respectable 54–28 record and won the division title. The Suns were again the highest-scoring team in the league with seven players averaging double figures in points per game, and Nash was voted for the first time to start for the 2006 Western All-Star team. Having recorded career highs in points (18.8), rebounds (4.2), field goal percentage (.512) and free-throw percentage (a league-leading .921), and leading the league with 10.5 assists per game, Nash was named the league MVP for the second year in a row. In the first round of the playoffs, Phoenix overcame a 3–1 deficit against the Los Angeles Lakers and won the series 4–3. The Los Angeles Clippers were their Conference Semifinals opponents, and the Suns again needed seven games to clinch the series. For the second year in a row, however, the Suns bowed out in the Conference Finals, this time to Nash's former team, Dallas.

A two-time NBA MVP, Nash is only the second point guard (along with Magic Johnson) to win the MVP award multiple times and the third guard in NBA history to earn back-to-back MVPs (joining Johnson and Michael Jordan). Only ten other NBA players have won back-to-back MVP awards: Johnson, Jordan, Bill Russell, Wilt Chamberlain, Kareem Abdul-Jabbar, Moses Malone, Larry Bird, Tim Duncan, LeBron James, and Stephen Curry. On 11 May 2006, ESPN.com rated Nash as the 9th-best point guard of all time, and in a survey by nba.com in 2007, Nash received 85% of the votes by the league's general managers as best point guard in the league. In a similar survey in 2009, Nash was rated as the best passer of the ball and the player possessing the best basketball IQ.

In 2007, Nash wrote and produced an 81-second commercial for Nike titled "Training Day", directed by Julian Schnabel's daughter Lola, which gained popularity as a viral video on YouTube. Nash also started a film production company together with his cousin, filmmaker Ezra Holland, and intends to produce independent films. The first creative effort to come from Meathawk was a 91-second commercial, titled "The Sixty Million Dollar Man", for Nike's eco-friendly Trash Talk shoe, the first high-performance shoe to be made—at the behest of the environmentally conscious Nash—from recycled materials. Nash has worn the shoe since February 2008 but Nike produced only 5,000 pairs for sale. The ad which broke virally on Earth Day 2008, was written by Nash and the Directors of the spot, Danny Vaia and Ezra Holland. It is a spoof remake of the title sequence of the American television series The Six Million Dollar Man and plays on Nash's numerous on-court collisions. Amar'e Stoudemire and Raja Bell have cameo appearances. Nash and Holland also co-directed the documentary Into the Wind, about iconic Canadian athlete and Activist Terry Fox, as part of ESPN's 30 for 30 series. In October 2013, Nash appeared in the music video for "City of Angels" by Thirty Seconds to Mars.

Before the 2008–09 season began, coach D'Antoni was replaced by Terry Porter, who preferred a more defensive-oriented style of basketball. The Suns had difficulties adapting to this new system, and even a December trade involving sending stalwarts Raja Bell and Boris Diaw to the Charlotte Bobcats for athletic swingman Jason Richardson saw the team continue to struggle. Porter was then replaced by Alvin Gentry in February after a 28–23 record, but the Suns were unable to secure the final seed for the playoffs, resulting in Nash missing the playoffs for the first time since he returned to Phoenix for his second stint.

Nash, along with former Yahoo! President and fellow Victoria-native Jeff Mallett, are Investors in Women's Professional Soccer, a soccer league that was launched in March 2009. Nash cited his twin daughters and wanting to have role Models for them to look up to as a reason for supporting the league. Nash also co-hosted Showdown in Chinatown in 2008, an 8-on-8 charity soccer game held at Sara D. Roosevelt Park. He scored two goals in his team's 8–5 victory. Participants included Thierry Henry, Jason Kidd, Baron Davis, and Suns teammates Raja Bell and Leandro Barbosa.

For the 2010 Winter Olympics held in Vancouver, Nash became the first NBA player in Olympic history to carry the torch and light the Olympic cauldron.

Nash had also previously made statements about his intention to bring Major League Soccer to Vancouver as early as 2011, which he has succeeded in doing. He joined the USL-1 Vancouver Whitecaps FC team's ownership group in July 2008 and in March 2009, Vancouver was officially named as a Future MLS expansion city, set to join the league in 2011. Nash occasionally attends practice for his co-owned team, also called Vancouver Whitecaps FC.

On May 8, 2012, Nash became General Manager of the Canadian senior national team. Three months later, he rehired Triano as head coach.

In July 2013, Nash participated in a training session with the Italian soccer club Inter Milan at the New York Red Bulls' facilities in New Jersey. He also trained with the New York Cosmos B of the American fourth-division National Premier Soccer League in 2015.

Nash grew up playing soccer—he stated in a 2005 interview that he could have played professionally if he had focused on it—and continues to hold an interest in the sport. When Dirk Nowitzki arrived in the NBA from Germany, he and Nash became close friends, in part because they enjoyed watching soccer together. Nash is friends with several professional soccer players, including Alessandro Del Piero, Thierry Henry, Owen Hargreaves, Massimo Ambrosini and Steve McManaman. During his off-season, when he lives in New York City, he has trained with the New York Red Bulls of Major League Soccer, and once tried to arrange a pick-up game in the city's Central Park with the Red Bulls and one of his local teams.

On September 25, 2015, it was confirmed that Nash would take on part-time consulting duties for the Golden State Warriors. During his first season with the team, the Warriors produced a record-breaking 73–9 season, although the team fell short in the 2016 NBA Finals to the Cleveland Cavaliers. The next season, the Warriors would win the 2017 NBA Finals against the defending champions Cleveland Cavaliers, giving Nash his first NBA championship in any role.

On January 5, 2016, it was announced that Nash was part of a group that purchased a $21 million stake in Spanish Segunda División club RCD Mallorca. The group also included Phoenix Suns owner Robert Sarver and Suns vice chairman Andy Kohlberg.

On December 19, 2017, the Naismith Basketball Hall of Fame announced that eligibility for induction into the Hall of Fame was decreased to three years after retirement, which allows for Nash to be eligible to be enshrined in 2018. On March 31, 2018, during the Final Four, Nash would be joined alongside former teammates Jason Kidd and Grant Hill, as well as Ray Allen, Maurice Cheeks, and Charlie Scott as the former NBA players that would enter the Basketball Hall of Fame for 2018.