In the 2018/19 off-season, Grimes underwent minor finger surgery but did not miss any training as a result. In mid-January 2019 he suffered a broken nose during a training session collision with teammate Nathan Broad. Grimes immediately returned to training after surgery to realign the nose, wearing a protective face mask for more than a month while the injury continued to heal. He played in each of the club's two pre-season matches before opening the season with a round 1 win over Carlton. Prior to the match, Grimes had been labelled by Fox Footy commentator Nick Riewoldt as "one of the best lockdown defenders in the competition" and highlighted as one of the league's most irreplaceable players. It was Richmond backline leader and five time All-Australian Alex Rance who would ultimately need to be replaced however, after suffering a torn ACL in that match which would rule him out for the entirety of the season and leave Grimes to lead the club's backline group. In round 2, Grimes received a one match suspension for striking Collingwood forward Jamie Elliott. When he returned in round 4, AFL Media named Grimes a key candidate to captain the side in the absence of all three members of Richmond's leadership group, though he would eventually be passed over for the role in favour of veteran Shane Edwards. He lead in another way, taking six marks in the final quarter in a best-on-ground performance that earned him his first three Brownlow Medal votes and that senior coach Damien Hardwick said "won the match for (Richmond)". Following that match, AFL Media labelled Grimes the best one-on-one defender in the league and that he had taken the mantle of the league's best defender from the injured Rance. He picked up two votes in the coaches association player of the year award in round 6 before adding more votes in three consecutive matches between round 8 and 10. In that round 8 win over Fremantle, Grimes gathered a career-best 26 disposals. Halfway through the year, Grimes was named in ESPN's mid-year All-Australian team and named by Fox Footy as a contender for the end of year honour. In round 18, Grimes earned two coaches votes for a performance in a win over Port Adelaide that also saw him named to AFL Media's Team of the Week. He was again named in that team the following week but did so along with best on ground honours with nine coaches votes in round 19's win over Collingwood. For that effort which included an equal career-high six intercept marks, Grimes was labelled by the Herald Sun as a likely All-Australian selection. In the five matches between the bye and that point, Grimes had lost just two of 21 one-on-one contests, while averaging a loss percentage of 16.4 across the length of the season. He was also picked by 3AW commentator Matthew Lloyd as the best key position defender in the league. During a match against West Coast in round 22, Grimes left the field with an ankle injury after landing awkwardly in a marking contest. He ultimately played out the match and despite some limited training during the week that followed, lined up in round 23's win over the Brisbane Lions where he restricted Charlie Cameron to just seven disposals and earned another selection to AFL Media's team of the week. Grimes concluded the regular season ranked 10th in the league for one percenters and 12th for intercepts, earning selection in The Age and the Herald Suns teams of the year. He earned an official honour with selection to the All-Australian team for the first time and was voted by the players association as the league's most courageous player. After nine straight wins to finish the home and away season, Grimes was set for another contest directly opposed to Charlie Cameron in a qualifying final against the Brisbane Lions at The Gabba. He produced a "dominant performance" to restrict Cameron to a single goal, helping Richmond win through to a home preliminary final against Geelong. Grimes received four coaches votes as the third best player on the ground during the qualifying final, but was also fined $1000 by the league for staging for a free kick when pushed by an opposition player in an off-the-ball incident in the first half of that win. In the preliminary final, Grimes was reliable in defence and helped his side through to a second grand final appearance in three seasons. In the grand final, Grimes held Coleman Medalist Jeremy Cameron to a single goal, and was part of the Richmond defence that kept the Giants to 25 points, their lowest score in the club's eight-year history and the lowest score by any team in a VFL/AFL grand final since 1960. Grimes finished what AFL Media called "a career-defining year" by winning a second AFL premiership and placing third in the club's best and fairest count, his best finish to date. He was also named as the league's best defender and 20th best player overall in the Herald Sun chief football reporter Mark Robinson's list of the league's best players in 2019.