Nelthorpe played for Brodsworth Miners Welfare and Frickley Athletic before joining Doncaster Rovers in 2004 and he made his first-team debut in 2005.

[10] His first appearance of the 2005–06 season came after being introduced as an 80th-minute substitute for Jonathan Forte against Cambridge United in the Football League Trophy, which Doncaster lost 3–2.

[11] He joined Conference North club Hucknall Town on work experience on 23 November 2005 and he made his debut in a 4–0 victory over Redditch United, during which he scored two goals.

[11] Nelthorpe signed a professional contract with Doncaster on 28 June 2006[16] and joined Conference National club Kidderminster Harriers on a one-month loan on 10 October.

[19] He scored his first career goal against Droylsden in the FA Cup, and after Kidderminster decided against extending his loan spell, he returned to Doncaster in November after making six appearances.

[24] After making four appearances for Doncaster during the 2007–08 season,[25] he joined Conference Premier club Halifax Town on a one-month loan on 10 January 2008.

[32][33] He was released by Doncaster at his own request on 13 January 2009 and joined Conference Premier club Oxford United the next day on a contract until the end of the 2008–09 season.

[37] Nelthorpe was released by Oxford at the end of the season and he subsequently signed for fellow Conference Premier club York City on 19 May 2009.

[43][44] He suffered from illness during much of York's pre-season, and during a friendly against FC Halifax Town he was sent off for jumping dangerously with his arm raised.

[59] Nelthorpe's first goal came after scoring a 90th-minute winner in a 1–0 victory over Southport,[59] turning in Josh Gillies' cross from six yards.

[62] However, he was released by the club in November alongside defender Rory Coleman, with manager Steve Housham claiming "they haven't lived up to expectations".

[68] His debut came on 24 March 2012 as a 72nd-minute substitute in a 1–0 defeat at home Marine, and was sent off after nine minutes for a needless challenge on Neil Harvey.

[74][75] Nelthorpe returned to England later that year, and re-joined Matlock in October 2015, scoring the winning goal less than a minute into his debut in a 1–0 home victory over Rushall Olympic.

[80] Nelthorpe signed for Northern Premier League Division One East club Belper Town on 17 August 2018.

[81] He had made 19 appearances and scored 1 goal in all competitions[82] by the time the 2019–20 season was abandoned and results expunged because of the COVID-19 pandemic in England.