|Date of birth||May 22, 1983|
|Place of birth||Trelew, Argentina|
|Height||5 ft 4 in (1.63 m)|
|San Lorenzo de Almagro|
|2008||→ D.C. United (loan)||7||(0)|
|2008–2009||→ Gimnasia La Plata (loan)||31||(6)|
|2011||→ Querétaro (loan)||18||(3)|
|2013||Gimnasia La Plata||18||(5)|
|* Senior club appearances and goals counted for the domestic league only and correct as of 29 July 2014.
† Appearances (Goals).
Niell joined the Argentinos Juniors youth system in 2001, and debuted professionally on April 12, 2004. In his first match he came on as a substitute for Gustavo Oberman, and scored a goal in the 3–1 victory over Colón de Santa Fe.
On January 22, 2008, he signed on loan with D.C. United. His first appearance for United was against Harbour View FC in the 2008 CONCACAF Champions Cup. On June 26, 2008 he was waived by D.C. United.
On July 12, 2009, coming from the bench in the 18 m. of the ST, he scored arguably two of the most important goals of his career when he scored 2 goals with his team Gimnasia La Plata in the last minutes of the game (44 & 46 ST), goals that allowed Gimnasia to remain in the first division of the Argentinian League. Gimmnasia was playing with 9 men and needed 2 goals to keep the category against Atlético de Rafaela, team that did beat them 3–0 in the first match. The two goals were headers.
- "D.C. United Snap Up Tiny Argentine Niell". Goal.com. January 8, 2008. Retrieved March 19, 2008.
- Goff, Steven (January 9, 2008). "Niell Close To Joining D.C. United". Washington Post. p. E02.
- D.C. United: News: Article
- Clarin: News: Article in Spanish