[7][8] As of 2021, Nelles is listed as 5 ft 11 in (1.80 m) and weighs 81 kg (179 lb) according to her biographies on the websites of Rugby Canada and McMaster Marauders.

[12] Nelles made her debut at the international level for Canada at the four-game 2011 Under 20s Nations Cup in Santa Barbara, California, placing third out of four teams with a 2–2 record.

This placed her seventh in the Ontario University Athletics player rankings, and was named to the OUA Russell Division All-Star Team and as a CIS All-Canadian selection.

Nelles was employed as head coach for the senior girl's rugby club of Westdale Secondary School for its spring season.

[3] She played club rugby for a short period before joining defending Farah Palmer Cup winners Canterbury.

[12] On October 31, 2020, the final of the 2020 Farah Palmer Cup, Nelles helped Canterbury win its fourth provincial title in succession when she scored a match-winning try on 81 minutes to beat Waikato 8–7 in Christchurch.

[17] At the international level, she made her debut for the Canada Maple Leafs senior women's developmental national team as captain on a two-match tour of England.

[8] Nelles was named as one of 25 players to Canada's 2017 Can Am rugby team for the series held in San Diego after doing a one-week training camp in Vancouver Island.

[20] She went on to play three matches for Canada at the 2017 Women's Rugby World Cup, scoring a single second-half semi-final try against Wales to finish in fifth position.

Cindy was a key leader and lineout specialist who helped the team become champions after defeating Chiefs Manawa 33–31 in the final.