As a young boy, Nielsen would sneak in to watch football games without paying, thus becoming a
gratist (free rider), abbreviated to "Tist". Born in
Copenhagen, Nielsen started playing football with
Kjøbenhavns Boldklub (KB), where he spent his entire senior career. He made his debut for the
Denmark national team on 5 May 1910 as the then youngest player, at 18 years and 131 days of age (exceeding
Vilhelm Wolfhagen's age record from 1908). Nielsen's record would remain for eight years, until it was surpassed by
Valdemar Laursen's debut in October 1918 at the age of 18 years and 51 days old. Nielsen made his debut in a Denmark national team that was playing its first game since winning silver medals at the
1908 Summer Olympics, and he was included in the Danish squad for the
1912 Summer Olympics tournament. He played one game at the 1912 Olympics, the 4–1 win against
the Netherlands, where he scored his first national team goal in his third national team game. Denmark later successfully defended their Olympic silver medal, losing 2–4 to
Great Britain in the final game, without Nielsen in the team. After the 1912 Olympics, Nielsen started scoring goals for the Denmark national team at a record-setting pace, while guiding KB to two Danish championships in a row during 1913 and 1914. He scored 22 goals in his first nine national team games after the 1912 Olympics, from May 1913 to June 1916, including six goals in a 10–0 win against
Sweden and all four Danish goals in a 4–1 win against
Germany. His tally in June 1916 was 23 goals in 12 national team games. He was a part of the KB team that won the 1917, 1918, 1922 and 1925 Danish championships, making 201 appearances for the club during this time. During the
1910–11 football season, he scored 47 of KB's 63 goals over the course of 10 matches, winning the Copenhagen championship. His body was laid to rest at
Bispebjerg Cemetery at his request. == Career statistics ==