In 2020, at age 29, he earned a move to French club Guingamp, but returned to Denmark after one season, signing with Silkeborg.

He never made a senior appearance for Lyngby, but gained several caps for Denmark national youth teams during his time at the club.

[8][9] Then director of sports in AaB, Lynge Jakobsen stated in the club's press release that "Nicolai is young, but has the potential to become a Superliga goalkeeper, and with us he will, among other things, through daily practice with Karim Zaza and our goalkeeping coach Poul Buus have the opportunity to continue his development".

[19][20] At the age of 20, Larsen signed a four-year contract extension with AaB keeping him a part of the club from Aalborg until 2015.

[38] To kick off the new season, Larsen appeared in two UEFA Europa League qualifiers on 18 and 26 July 2014 against Georgian club Dila Gori,[39] where AaB lost 3–0 on aggregate.

[42][43][44] It had originally been planned that the award should have been presented to Larsen prior to the match against Brøndby IF on 22 February 2015 by Troels Bager Thøgersen, editor-in-chief of Tipsbladet,[45] but Larsen had to postpone in the last minute due to illness, and therefore the award was presented to him at the match against Randers FC on 8 March 2015.

It is a personal title, and it makes it is awesome that my competitors and colleagues are the ones who vote [...]" and he continued "[t]his is one of those awards I will look back on and be proud of[...]".

[47] On 31 August 2017, the last day of the summer transfer window, Larsen moved to Superliga club FC Nordsjælland.

[50] He made his official debut for the club in the 2017–18 Danish Cup 3rd round matchup against Vejgaard BK, where Larsen kept a clean sheet, as Nordsjælland won 4–0 on Soffy Road.

[54] Runarsson was sold to French club Dijon FCO in June 2018,[55] and in early July 2018, Nordsjælland brought in Nigel Bertrams from Dutch side NAC Breda.

[58] In particular Larsen was praised for his performances in the first leg of the second round qualifier against Swedish club AIK, where he, among other things, saved a penalty kick.

[60] On 25 June 2020, after three years at Nordsjælland, Larsen was signed by French club En Avant Guingamp on a two-year contract.

[65] On 23 June 2021, it was announced that Larsen had signed a two-year contract with newly promoted Danish Superliga club Silkeborg IF.

[66] He made his debut on 19 July – the first matchday of the 2021–22 Danish Superliga – keeping a clean sheet in a 0–0 home draw against SønderjyskE.

[76][77][78] In the match against Turkey, Larsen did not make an appearance as Denmark lost 2–1 on goals by Ozan Tufan and Olcay Şahan.

[86] Carsten Larsen has been described as a responsive goalkeeper with good hands, but stopped prematurely in 1999 after a series of serious cruciate ligament injuries.