During the 2017–18 season, Rielly was injured in a game against the Philadelphia Flyers on January 19, 2018, and was placed on injured reserve on January 22. He was back in the Leafs' lineup on February 3, 2018, after missing six games. Despite his injury, Rielly and fellow defenceman Jake Gardiner became the first two Leafs defenceman to record at least 50 points in a season since Tomáš Kaberle and Bryan McCabe accomplished the feat in 2006–07. He ended the regular season with a career-high 46 assists and 52 points, tying Gardiner for the team points leader amongst Leafs defencemen, and helped propel the Leafs to their second consecutive playoff showing. Despite pushing the Boston Bruins to seven games, the Leafs lost the series in the first round, as Rielly finished the playoffs with five points.
During the 2018–19 season, on October 9, 2018, Rielly recorded a career-high four assists in a 7–4 win over the Dallas Stars, bringing him to ten points in his first four games, tying him for the NHL lead in points alongside teammate Auston Matthews. The following game, on October 11 against the Detroit Red Wings, Rielly passed Bobby Orr for most points by a defenceman in a team's first five games, with 12 points. By picking up an assist the next game, he established the longest season-opening point streak by a defenceman in franchise history. On October 14, Rielly was named as one of the NHL's 3 Stars of the Week for the first time in his career. He had an NHL-leading six assists, along with an overtime goal against the Chicago Blackhawks, during a four-game Maple Leafs win streak. He was named as the NHL's second star of the week for his accomplishments. On December 20, during a 6–1 Leafs win over the Florida Panthers, Rielly recorded his second four assist game of the season, becoming the first NHL defenceman that year to reach the 40-point mark. On December 24, Rielly was again named as the 2nd star in the NHL's 3 Stars of the Week. Rielly led the NHL with 3 goals, 6 assists, 9 goals and a +10 rating the previous week to propel the Maple Leafs to a perfect week and into second place in the overall League standings. On January 23, 2019, Rielly became the first Maple Leafs defenseman to post consecutive 50-point seasons since Tomas Kaberle's 2005-2006 and 2007-2008 seasons. Rielly reached the 50-point mark while skating in his 49th personal game of 2018-19. Only three other Maple Leafs defensemen have reached the mark faster: Borje Salming (42 GP in 1976-77), Babe Pratt (44 GP in 1943-44, and Bryan McCabe (45 GP in 2005-06). He scored his 20th goal of the season on March 16, 2019 against the Ottawa Senators. With his 20th goal of the season, Rielly became the first Maple Leafs' defenceman to score 20 goals in a season since Al Iafrate in the 1989-90 season. Rielly ended the season with a career-high 20 goals, 52 assists, and 72 points, the first 70+ point season by a Maple Leafs' defenceman since Börje Salming in 1980. He was one of only three defensemen under 25 to score 50 points or more, along with Thomas Chabot and Jacob Trouba.