[1] Nieto grew up in Long Beach, California,[2][3] in a dangerous neighborhood; his mother had stated: "There were drive-by shootings.

During his time in the program he and three other players including Jon Merrill were suspended for sexual harassment and lewd behaviour.

[13] Nieto was ranked the 43rd-best prospect and the second-best player in the NCAA's Hockey East conference by NHL.com among the 2011 draft class.

[19] He scored his first career NHL goal against Henrik Lundqvist of the New York Rangers on October 8, off assists by Joe Pavelski and Tommy Wingels.

[22] Despite being reassigned to the San Francisco Bulls of the ECHL on December 18,[23] Nieto was recalled by the Sharks three days later after an injury to forward Tomáš Hertl.

[28] As a free agent for the first time in his career at the conclusion of the 2019–20 season, Nieto returned to the San Jose Sharks in signing a one-year, $700,000 contract on October 13, 2020.

He made his 500th NHL career appearance in a game against the St. Louis Blues on March 19, 2021, becoming just the third California native to reach the mark behind Brooks Orpik and Jason Zucker.

With the Sharks out of contention for a playoff berth, Nieto was traded alongside Ryan Merkley in a return to the Avalanche, in exchange for Martin Kaut and Jacob MacDonald on January 25, 2023.

[32][33] He completed the season combining his offensive output between the Sharks and Avalanche to finish with 12 goals and 24 points, both the second highest totals of his career.

As a free agent, Nieto left the Avalanche to sign a two-year, $1.8 million contract with the Pittsburgh Penguins on July 1, 2023.

[34] Nieto was first selected to represent the United States at the 2009 IIHF World U18 Championships, posting four goals in seven games to help capture the gold medal.