In 2016, he made a move to Skovshoved, a team competing in the fourth-tier Denmark Series. His debut for the club occurred on 10 August 2016, during a Danish Cup first-round match against BSF. In an impressive start, he scored a brace, securing a 4–1 victory for his team.[7] He made his league debut on 10 September, coming on as a substitute in the 77th minute in a 1–1 home draw against B 1908.[8] On 24 September, he scored his first league goal for the club, as Skovshoved lost 3–2 at home to AB Tårnby.[9] As a midfielder, Vallys played a significant role, contributing to the team's promotion to the 2nd Division. Across two seasons at Skovshoved, he showcased his talent, scoring 28 goals in 52 appearances. In June 2018, his skills caught the attention of FC Roskilde, leading to a two-year deal with the club.[10] Transitioning to the second-tier 1st Division, he maintained his impressive performance, scoring seven goals in 29 league appearances.[11]
Silkeborg
On 1 July 2019, Vallys signed a two-year contract with newly promoted Superliga club Silkeborg, continuing his rapid rise through the Danish divisions.[12] He made his debut for the club, as well as his debut in the highest Danish division, on 14 July 2019 in a 3–0 away loss to Brøndby IF, coming on as a substitute in the 72nd minute for Marc Rochester Sørensen.[13] He finished his first season with Silkeborg with 34 total appearances in which he scored six goals,[11] as the club suffered relegation to the 1st Division.[14]
During the summer of 2020, he suffered an injury while training with a friend, sidelining him for 2–3 months.[15] On 1 September 2020, he signed a contract extension with Silkeborg until December 2023.[16] He returned to the pitch in October 2020, and played a key role as Silkeborg won promotion back to the Superliga in May 2021.[17]
Brøndby
On 31 August 2022, it was confirmed that Vallys had joined Brøndby IF on a deal until June 2026.[18] Brøndby reportedly paid DKK 22.5 million (€3 million).[19] The move was regarded as controversial by some Brøndby supporters,[20] as he had supported bitter rivals FC Copenhagen as a youth and hails from their home district of Østerbro.[21][22] After Vallys' received threatening messages, director of sports Carsten V. Jensen, head coach Niels Frederiksen, and club captain Andreas Maxsø publicly criticised some fans' behaviour regarding the transfer.[20][23]
Vallys made his competitive debut for Brøndby on 4 September in a 2–0 league win away against AC Horsens at CASA Arena.[24] He scored his first goal for the club in the following league game against Randers FC, his home debut at Brøndby Stadium, slotting home the 2–1 lead in an eventual 2–2 draw.[25] On 19 February 2023, Vallys scored a hat-trick in Brøndby's spring opener against Horsens, which ended in a 5–2 victory.[26]
On 2 December 2023, Vallys was honoured with the Brøndby Player of the Year 2023 award. This recognition came after his remarkable performances throughout 2023, contributing 13 goals and five assists in 31 appearances and earning his first cap for the Denmark national team.[27]
^"Galleri: Brøndby IF-SIF". Silkeborg IF (in Danish). 14 July 2019. Retrieved 4 November 2020. Kampen på Brøndby Stadion gav debut til to af de nye i SIFs spillertrup, Nicolai Vallys og Filip Lesniak