Marner started his youth hockey career playing for the Clarington Toros of the Ontario Minor Hockey Association (OMHA) as a seven-year old, primarily playing with and against players a year older. After a year with the Toros, Marner moved to the Whitby Wildcats, also a part of OMHA. He spent two years playing for the Wildcats before transitioning to the Greater Toronto Hockey League (GTHL), where he joined the Vaughan Kings and went on to win a GTHL title.
For the 2012–13 season, Marner transitioned to the Don Mills Flyers of the GTHL where he played his minor midget year, registering 86 points in 55 games. He finished second in scoring in the GTHL behind Dylan Strome (who was then playing with the Toronto Marlboros). At the end of his season with the Flyers, he was immediately invited to join the St. Michael's Buzzers of the Ontario Junior A Hockey League, where he went on to win a championship.
During his minor midget season, Marner was initially unsure of his plans for the following season. He received a scholarship offer from the University of Michigan, while also being drafted by the London Knights in the first round of the 2013 OHL Priority Selection, 19th overall.
Despite receiving a scholarship offer from the University of Michigan, Marner elected to sign with the London Knights, who selected him with their first round pick in the 2013 Priority Selection.
Marner had a strong rookie season with the Knights, registering 59 points in 64 games and was the runner up for the OHL rookie of the year, behind Travis Konecny.
During the 2014–15 OHL season, Marner experienced superb play playing alongside line mate Max Domi. As the season progressed, Marner was consistently included as one of the top prospects in the 2015 NHL Entry Draft, being listed as a definitive top ten pick, if not top five. He would lead the league in scoring for most of the season until Dylan Strome of the Erie Otters recorded six points in the final game of the season, knocking Marner down to second. in reward for his outstanding sophomore season, he was named to the OHL First All-Star team and awarded the Jim Mahon Memorial Trophy as the OHL's highest scoring right wing player. Marner would end up being selected fourth overall by the hometown Toronto Maple Leafs.
On July 28, 2015, the Leafs signed Marner to a three-year entry level contract.
On October 1, 2015, Marner was named as co-captain of the Knights along with teammate, Christian Dvorak. Marner played most of the season at right wing on a line with Dvorak and Matthew Tkachuk. The trio was a dominant force and the Knights ended the regular season as the highest scoring team in the OHL. Marner himself finished the regular season second in league scoring with 116 points in 57 games and was awarded the Red Tilson Trophy as the OHL's most outstanding player of the year.
Marner played an important role during the Knights' 2016 playoff run, resulting in the team winning the OHL Championship. Marner led the playoffs in scoring and with 44 points in 18 games and was awarded the Wayne Gretzky 99 Award as the playoff MVP.
Marner helped lead the Knights in winning the 2016 Memorial Cup. He won both the Stafford Smythe Memorial Trophy and the Ed Chynoweth Trophy as tournament MVP and leading scorer.
After an impressive training camp and pre-season (in which he led the team with four assists), it was announced that Marner would remain on the Maple Leafs roster for the upcoming 2016–17 season. He made his NHL debut in the team's season opener on October 12. Despite an effective first game, he was overshadowed by a historic four goal debut from teammate Auston Matthews. The next game, Marner would score his first NHL goal. Marner would pick up his first career assist four days later. On October 27, 2016, in a game against the Florida Panthers, Marner had his first multi-point game, accumulating 3 assists to lead the Leafs to a 3–2 win. Marner would then have his first multi-goal game against the Buffalo Sabres on November 3, 2016 and the Leafs defeated the Sabres 2-1. On November 15, 2016, Marner had 3 points in a 6–2 win over the Nashville Predators, which, at the time, put him in the joint-lead for Leafs' leading scorer (with James van Riemsdyk) for the first time of his career. Marner also set the Maple Leafs record for most assists in a season by a rookie with 42 assists on the year. This broke the previous record which was held for 73 years by Gus Bodnar who had 40 assists in the 1943–44 season. Marner spent the entire season on a line with van Riemsdyk and Tyler Bozak, finding offensive chemistry with the two veterans, and managed to finish the year with 61 points. His play assisted the team in qualifying for the post-season. Marner picked up four points in the series but it was not enough as the team lost in six games against the top seeded Washington Capitals.
Marner made his international debut for Canada at the 2014 Ivan Hlinka Memorial Tournament, where he won a gold medal. He tied Mathew Barzal for the leading scorer from Canada at the tournament.
At the 2016 IIHF World Junior Championships held in Helsinki, Marner and Dylan Strome each scored four goals and 2 assists in 5 games to lead the Canadian team in scoring. Canada reached the quarterfinals, but was eliminated by the host Finland. Marner scored twice in the game, both times to bring Canada back to an even score, but it wasn't enough to overcome the eventual gold medalists.
Marner made his senior team debut at the 2017 IIHF World Championship, where team Canada won silver. Scoring 12 points in 10 games, Marner was placed in the top 10 in scoring and was only second to Nathan MacKinnon in team scoring.
Marner was born in Markham, Ontario and spent the majority of his life in the neighbourhood of Thornhill. His parents are Matt and Sydney Marner and he has a brother, Christopher, who is four years older. The Marners consider themselves an animal family; they currently own two pets, a cat named Burbank and a chocolate Labrador, Winston.
Marner attended Hill Academy, a private school focused on athletics, located in Vaughan. He would later go to Blyth Academy. At Blyth, Marner partnered with Knights teammates Christian Dvorak and Owen MacDonald to establish a café known as MOD Feast, with MOD being an acronym for each creators names (Marner, Owen and Dvorak). The group offered "bagels and stuff", with Fridays being designated to serving pizza.
Growing up, Marner was a fan of the Toronto Maple Leafs, the team that eventually drafted him. He was also a fan of the Pittsburgh Penguins and Chicago Blackhawks due to the presence of his two favourite players, Sidney Crosby and Patrick Kane. Alongside hockey, Marner has listed his favourite movie as Step Brothers, his favourite television show as The Big Bang Theory, and cited his favourite video games as the Grand Theft Auto and Call of Duty series'.