Junior Robertson played junior ice hockey for the
Edmonton Oil Kings of the
Western Hockey League (WHL) from 2017 to 2021. He was selected by Edmonton in the 2016
WHL bantam draft with the seventh overall selection. He played seven games for Edmonton in the
2016–17 season where he got to be a teammate of his older brother Tyler Robertson, who played forward with the Oil Kings. Going into the
2019 NHL entry draft Robertson was projected by many draft analysts as a possible first round draft choice. He was ranked 26th among North American skaters by the
NHL Central Scouting Bureau. Robertson said of being drafted "Just hearing my name get called, it's a dream I've had since a little kid. It's a surreal experience just going through it."
USA Today writer Vincent Mercogliano immediately rated Robertson as the Rangers' 10th-best prospect. Robertson returned to Edmonton for the
2019–20 season. He played 60 games recording 13 goals and 34 assists. After the season Mercogliano increased his rating to being the Rangers' seventh-best prospect. Pierce also said "He was mean. He was hard to play against. He really did start to play like a guy the size that he is with how quickly he can end plays and transition pucks." Mercogliano named him the Rangers' sixth-best prospect. After the season Mercogliano named him the Rangers' fourth-best prospect. Robertson said of the experience that "I learned a lot. On the defensive side – positioning, angles, just little things to be more efficient. ... On the offensive side, just getting shots through, knowing when to jump up and get into little pockets. I simplified my game." Robertson started the 2022–23 preseason with the Rangers but was assigned to the Hartford Wolf Pack before the season began. His
2022–23 season ended after 57 games when he suffered a shoulder injury. Although he improved offensively in 2022–23, Robertson committed too many turnovers and his defensive play was sometimes inconsistent. Robertson started the 2023–24 season with Hartford and was recalled to the Rangers three times, although he was returned to Hartford each time without playing in a game for the Rangers. After Hartford was eliminated from the
Calder Cup playoffs, Robertson was recalled to the Rangers roster for the
Stanley Cup playoffs. Prior to the 2024–25 season, Robertson received a qualifying offer from the Rangers at the end of his entry level contract despite having not yet played a game for the Rangers. During the offseason leading up to the 2024–25 season he worked with a mental skills coach to help with confidence issues that he felt had been holding his career back. Robertson started the 2024–25 season with Hartford, but was recalled to the Rangers on February 26. He was sent down before playing a game but was recalled again on April 13 and made his NHL debut for the Rangers the following day in a game against the
Florida Panthers. Robertson made the Rangers opening night roster to begin the 2025–26 season. He scored his first NHL goal on 18 October 2025 in a game against the
Montreal Canadiens. ==International play==