Dumitru started his professional career at Empoli, in a match during the 2008–09 season in Serie B. He also played another game in the following season, still at Serie B level.
On 31 August 2010, he was signed by Serie A club S.S.C. Napoli at age 18 and 11 months. He failed to break into the first team during his period at Napoli, making only nine Serie A appearances at the club. Dumitru also played a few games for the reserve.
Return to Empoli
On 20 July 2011, it was announced Dumitru would return to Empoli in Serie B for the 2011–12 season on a one-year loan basis. On 27 August he scored on his debut, in the first match of the league, against Juve Stabia.
Further Italian loan spells
On 23 July 2012, Dumitru joined Serie B club Ternana on a season-long loan deal until the end of the 2012–13 season. On 31 January 2013 the loan was terminated. Dumitru joined Citadella on a temporary deal the same day. He joined Reggina on loan for the 2013–14 season.
On 13 August 2014, Greek club Veria announced the loan signing of Dumitru from Napoli on a one-year deal. He debuted on 24 August 2014 in the season's premiere home match against Skoda Xanthi where he scored his first goal. On 13 September 2014, in a match against Kerkyra, he scored his second goal and was the victim of "derogatory and racist remarks" Horacio Cardozo. The incident was reported by Veria's squad and Cardozo was invited by the Hellenic Football Federation to further give an explanation of the incident that took place. The HFF found Cardozo not guilty due to the lack of strong evidence. Dumitru was twice nominated for the Best Goal award for the goals that he scored on the first matchday as well as on the third, and won the award for his second goal in the championship. He also scored twice on his 2014–15 Greek Cup debut in home 4–1 victory against Ermionida. He got his first assist with Veria against PAS Giannina where Nikos Kaltsas scored. Nicolao scored his third championship goal in a home victory against Levadiakos during Superleague's seventh matchday.
On 30 August 2016, Dumitru joined Nottingham Forest on a season-long loan. On 30 December 2016 he scored his first goal for the club in a 3–1 loss against Newcastle United.
On 25 July 2019, Nicolao Dumitru signed a two-year contract with Romanian club Gaz Metan Mediaș.
International career
Dumitru was eligible to play international football for Sweden by birth, Brazil through his mother, Romania through his father and Italy through residency, his family having migrated there from Sweden when he was a child. With the Italy U-19national team in 2010, he took part in the U-19 European Championship. He was also a member of the Italy U-20 national team.
Personal life
Dumitru was born in Nacka, Sweden to a Romanian father and an Afro-Brazilian mother, and successively moved to Empoli, Tuscany in 1998 with his parents.
