Born in Norrköping, Fransson joined IFK Norrköping as a 14-year-old from Lindö FF in 2008, and played in the club's academy until 2012. He made his Allsvenskan debut on the opening game of the 2013 season, in their away win against Mjällby AIF, the day before his 19th birthday. He made in total 20 appearances in 2013, 18 in the league and two cup games. His first Allsvenskan goal came also away at Mjällby, scoring the team's only goal in their 3–1 defeat on 14 August 2014. For IFK Norrköping he played as a midfielder. In the last round of 2015 Allsvenskan, on 31 October 2015, Fransson won his first senior title as IFK Norrköping won the Swedish championship for the first time in 26 years. Eight days later, Fransson and IFK Norrköping took their second title for the season when they defeated 2014–15 Svenska Cupen winners IFK Göteborg in 2015 Svenska Supercupen.
Basel
On 2 January 2016, FC Basel announced that they had signed Fransson on a four and a half year contract up until 30 June 2020. He made his league debut for Basel on 14 February 2016 in the 4–0 away win against Grasshopper Club coming in as substitute in the 90 minute. He scored his first league goal for Basel on 21 February during the 5–1 home win against Vaduz. Under trainer Urs Fischer Fransson won the Swiss Super League championship at the end of the 2015–16 Super League season and at the end of the 2016–17 Super League season. For the club this was the eighth title in a row and their 20th championship title in total. They also won the Swiss Cup for the twelfth time, which meant they had won the double for the sixth time in the club's history. On 26 December 2017 FC Basel announced that Fransson would be loaned to Lausanne-Sport to gain more first-team playing experience.
IFK Norrköping
On 4 July 2018, IFK Norrköping announced that they had signed Fransson on a three year contract until 30 June 2021.
International career
Fransson represented the Sweden U19 national team nine times between 2012 and 2013. In November 2014, Fransson played his first game for Sweden U21. Fransson made his debut for the Swedish national team in an unofficial friendly against Estonia on 6 January 2016, played in the Armed Forces Stadium in Abu Dhabi. He started the match and played 55 minutes before being substituted in the 1–1 draw. His second cap followed on 10 January in a 3–0 win against Finland played at the same venue. This match was also an unofficial friendly and he came on as a substitute in the 62nd minute.
