Lehner started playing hockey relatively late at ten years old. He had previously played soccer. He began playing hockey with Mölndal IF, and in only seven years he became one of the top Swedish goaltenders in his age group. Lehner played for Frölunda HC until the 2008–09 season. After being drafted by the Ottawa Senators in the second round, 46th overall, in the 2009 NHL Entry Draft, he moved to Canada to play with the Sault Ste. Marie Greyhounds of the Ontario Hockey League (OHL).
On 29 March 2010, Lehner signed a three-year, entry-level contract with the Ottawa Senators. Ottawa sports media suggested that the "highly touted prospect" would provide a measure of stability and longevity to the Senators' goaltending, something the organization has rarely had since its inception.
In 2010, following a successful season in Sault Ste. Marie, the Senators organization informed the team that Lehner would not be returning, as he would begin his time with the Binghamton Senators for the 2010–11 season. Lehner made his AHL debut on 15 October 2010. The next day, Lehner was called up to Ottawa due to an injury to Pascal Leclaire. He made his NHL debut playing four minutes in the third period of a game against the Montreal Canadiens that night. With that debut at 19 years, two months and 24 days, Lehner became the youngest Swedish goaltender to play in the NHL, breaking Jhonas Enroth's record of 21 years, four months and 13 days.
Lehner won his first NHL start when Ottawa defeated the New York Islanders 6–4 on 13 January 2011. On 7 June 2011, Lehner was awarded the 2011 Jack A. Butterfield Trophy as the AHL's Calder Cup Most Valuable Player with the champion Binghamton Senators.
On 17 May 2013, Lehner made his Stanley Cup playoff debut against the Pittsburgh Penguins, stopping 20 of 21 shots in relief of starter Craig Anderson.
The 2013–14 season saw Lehner start the season in Ottawa for the first time after three years of tutelage in the AHL. "I’ve lived with my suitcase from hotel to hotel. Now you have a base and you can start something," he said of finally being a full-time NHL player. Observers began suggesting that he may be a future franchise-goalie in the NHL. On 11 November 2013, after posting a 3–0 record with a 1.33 goals against average (GAA) and .958 save percentage, Lehner was named the NHL's First Star of the Week.
On 31 July 2014, Lehner signed a three-year deal with the Senators worth $6.675 million.
On 26 June 2015, Lehner and David Legwand were traded by the Senators to the Buffalo Sabres in exchange for the 21st overall pick in the 2015 NHL Entry Draft (Colin White).
Lehner has competed internationally for Sweden in the 2009 IIHF World U18 Championships in Fargo, North Dakota, and Moorhead, Minnesota, where Sweden placed fifth. He competed for starting goaltender at the 2011 World Junior Ice Hockey Championships in Buffalo, New York.
Lehner is a butterfly-style goalie who uses his large frame to his advantage. Despite his size, he remains quite mobile with good lateral movement. Following the 2009 NHL Entry Draft, Senators' scouting staff described Lehner's style as very competitive with good technique and a willingness to aggressively challenges shooters. Team Sweden Head Coach Stephan Lundh described him as a "steady, calm person. He's very determined."
Robin's father Michael has helped coach New York Rangers' goaltender Henrik Lundqvist, and has had a big influence on both goaltender's playing styles. His father never played goaltender, so he would often use his son as a "guinea pig" for Lundqvist. Michael Lehner has done various forms of martial arts at an elite level and took his knowledge of movement over to goaltending. Michael Lehner is currently the goaltending coach for Frölunda HC, Lundqvist's and Robin Lehner's former team.
When longtime Senators' captain Daniel Alfredsson signed with the Detroit Red Wings in 2013, he sold his house in Ottawa to Lehner. Lehner is a fan of melodic death metal and in particular Swedish band In Flames, who originate from Lehner's home town of Gothenburg. The band influenced the design on Lehner's mask.
Lehner and his wife, Donya Lehner (née Naini) welcomed their first child, son Lennox, on 30 October 2014. Donya is of Persian decent. Politically, Lehner supported Donald Trump during the 2016 presidential election.OHL Goalie of the Week ( 16–22 November 2009)
OHL Goalie of the Week ( 25–31 Jan 2010)
Recipient of Jack A. Butterfield Trophy, AHL Playoff "Most Valuable Player" (2011)
Calder Cup Champion, Binghamton Senators (2011)