Icelandic Under-16 international Isak Thorvaldsson signed for Norwich on 21st July 2017 as a first year scholar. He was previously with Icelandic third tier side Afturelding

“Thorvaldsson is a box-to-box midfielder, with a good goal-scoring record,” said Norwich's Head of Academy Recruitment Gregg Broughton "He has been selected on a regular basis for Iceland at U16 and U17 level, and will represent his country at the Nordic Cup in August."

He played for Norwich in the Foyle Cup in Northern Ireland in July 2017, helping the Canaries reach the final before losing 1-0 to Kildrum.

In August 2017, he was selected for the Iceland Under-19 squad.

He signed his first professional contract with City on 20th February 2019. He has also captained Iceland's Under-19 side.

On 31st January 2020,he was loaned to League One side Fleetwood until the end of the 2019/20 season. Also joining Fleetwood on loan were  Everton's Lewis Gibson and Swansea City's Barrie McKay. He made two appearances for the Cod Army before the Covid-19 pandemic ended football prematurely,

On 16th July 2020 it was announced that Isak had signed a contract extension until 2022. Four days later he joined Scottish Premiership side St Mirren on a season long loan following the route of Akin Famewo. He came off the bench during St Mirren’s opening two Scottish Premiership games of the season, playing the final 13 minutes of a 1-0 home win over Livingston on the opening day and the final six minutes of a 3-0 loss at Rangers. However, he remained on the bench during a 1-0 win at Hamilton and was not in the squad for a 1-1 home draw with Ross County. The loan was then ended early,

On 27th August 2020 he returned to Iceland joined IA (Iprottabandalag Akraness) on loan for the remainder of the season. IA are managed by former Iceland, Burnley and Huddersfield midfielder Joey Gudjonsson. Thorvaldsson scored three goals and registered 3 assists as he helped IA survive against the drop by a single point. 

On 20th October 2021 it was reported that Isak was on trial at Scottish side Livingston ahead of a potential loan move in January 2022.

On 1st January 2022 Isak confirmed his departure from Carrow Road. He posted on Instagram that his five years at Norwich had come to an end and that it is time for him to take the next step in his career elsewhere.