Born in Odense, Denmark, Hermansen began his youth career at Næsby.[3] As a ten-year-old, he transitioned from being an outfielder into a goalkeeper after suffering from achilles tendinitis.[3]
In September 2015, Hermansen moved to Brøndby.[4] He was promoted to the first team prior to the 2019–20 season.[5]
On 5 November 2020, Hermansen made his professional debut in a Danish Cup match against Ledøje-Smørum, which ended in a 1–0 win for Brøndby.[6] At the end of the 2020–21 season, Brøndby became Danish champions for the first time since 2005,[7] with Hermansen featuring as the backup to German goalkeeper Marvin Schwäbe. As a result, he did not make a league appearance.[8]
After Schwäbe's departure to 1. FC Köln, Hermansen assumed to role as starting goalkeeper.[9] He made his debut in the Danish Superliga on 18 July 2021 in a 1–1 draw against AGF.[10] The following month, on 17 August, Hermansen made his European debut in a 2–1 loss to Red Bull Salzburg in the first leg of the play-off round of the UEFA Champions League.[11] Despite the loss, Hermansen impressed in goal with vital saves.[12][13]
On 3 December 2022, Hermansen received the award of Brøndby Player of the Year 2022 after putting in strong performances through the year.[14]
Leicester City
On 18 July 2023, Hermansen signed for Championship club Leicester City on a five-year deal.[15] He made his competitive debut for the Foxes on the opening matchday of the season, providing crucial saves to help secure the team a 2–1 league victory against Coventry City.[16] However, he would pick up a quad injury during training a few days later, sidelining him for the following league game against Huddersfield Town.[17] He returned to the starting lineup on 19 August in a league win against Cardiff City.[18] On 20 September, he kept his first clean sheet after joining the Foxes, helping his side to a 2–0 away victory against Norwich City at Carrow Road.[19] After the game, Hermansen discussed his role as ball-playing goalkeeper in manager Enzo Maresca's team, saying: "He (Maresca) gives me a lot of responsibility and I really appreciate that ... I like to try to dictate the game. As a lot of teams do, they leave me open and play man-to-man with the rest of the guys, so it's up to me to find these smaller passes, to try to move things around and create these spaces."[20] His strong form throughout the season earned him a place in the EFL Championship Team of the Season, as Leicester won the division title and secured promotion back to the Premier League.[21][22]
Hermansen is renowned for his ball-playing skills, alongside shot-stopping talent.[29] He is also considered a specialist in saving penalties, having done so in crucial occasions including a UEFA Europa Conference League qualifier against Basel in 2022 and a derby against Copenhagen,[30][31] as well as a triple penalty save against FC Nordsjælland in 2023.[32]