Amateur Nieto began attending
Salisbury School, a preparatory school, in
Salisbury, Connecticut, at the start of his second year of high school. In one season with the school's hockey team, he scored eight goals and ten assists in 28 games. The performance drew the attention of the
USA Hockey National Team Development Program, and Nieto subsequently moved to
Ann Arbor, Michigan, to play in the Program. In 2008–09, he played for the under-17 and under-18 teams, scoring 20 goals and 33 assists and six goals and eight assists, respectively. The next season, he played for the Program's
United States Hockey League (USHL) team, scoring 15 goals and 14 assists; he also returned to the under-17 team, scoring one goal. After participating in the Program, Nieto was offered a chance to play
major junior hockey for the
Tri-City Americans of the
Western Hockey League (WHL), but opted to attend college instead and play at the
collegiate level. When choosing colleges, Nieto decided between
Boston University and
Boston College, 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. Hockey's Future stated he had struggled earlier in his career defensively, but had since improved. They added he had "explosive speed", and excels with puck handling and passing while moving. The website gave him a 7.5/10 talent score. Nieto became the first Californian to be selected by the team. He spent time with the Sharks' then-
American Hockey League (AHL) affiliate, the
Worcester Sharks, during the year, playing in 11 games. He scored two goals with four assists, a −4 plus-minus rating and six penalty minutes. Nieto made his NHL debut against the
Vancouver Canucks on October 3, 2013. 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. Nieto, however, returned to Worcester over one month later, on November 25, though he was later recalled on December 7. Despite being reassigned to the
San Francisco Bulls of the
ECHL on December 18, Nieto was recalled by the Sharks three days later after an injury to forward
Tomáš Hertl. Nieto scored his first career
Stanley Cup playoff goal on April 22, 2014, via an
empty net goal against the
Los Angeles Kings. In the
2016–17 season, Nieto struggled to retain his role in adding an offensive touch to the Sharks' bottom six forwards. Surpassed in the depth chart, Nieto played in just 16 games with 2 assists before he was placed on waivers in order to gain more playing time. Nieto was claimed off waivers by the
Colorado Avalanche the following day, on January 5, 2017. On July 25, 2017, Nieto and the Avalanche avoided arbitration by agreeing to a one-year, $1 million contract. Nieto set career high's in the 2018 season with 74 games played and 15 goals scored. After the year the Avalanche signed Nieto to a two-year deal worth $3.95 Million in July 2018. 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. In the following season, Nieto contributed to the Sharks forward depth, adding 5 goals and 7 points in 25 games before his season was derailed through injury. 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. in 2020. On June 21, 2021, Nieto agreed to a two-year, $1.7 million contract extension to remain with the Sharks. In the season, having returned to full health, Nieto posted 6 goals and 17 points through 70 regular season games, as the Sharks missed the post-season for the third consecutive season. In his final year under contract with the Sharks in the season, Nieto showed his versatility amongst the Sharks forward-lines, increasing his scoring output when elevated alongside
Logan Couture in scoring 8 goals and 15 points through 48 regular season games. 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. 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. He was scoreless in 7 playoff contests with the injury struck Avalanche in a first-round defeat to the
Seattle Kraken. 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. ==International play==