In his first full season in 1984, San Diego won their first NL West title. Gwynn was elected to start in his first All-Star Game, and he won his first batting title with a .351 average along with 71 RBIs, and 33 stolen bases; he had only 23 strikeouts in 606 at-bats. He finished third in the balloting for the National League MVP behind Chicago Cubs winner Ryne Sandberg and runner-up Keith Hernandez of the New York Mets. He had 213 hits, breaking the Padres record of 194 set by Richards in 1980. Gwynn hit second in the Padres batting order behind Alan Wiggins, and benefited from the higher number of fastballs opposing pitchers threw in response to the speedy leadoff hitter (70 SB) being on base. Wiggins' speed also disrupted the defense and opened up holes that Gwynn was able to exploit for hits. He hit above .400 when Wiggins was on base ahead of him. "Anybody can hit a fastball", said Gwynn. The duo was one of the biggest reason behind San Diego's success. They could score fast with Wiggins getting on first, stealing second, and Gwynn singling him home. Gwynn batted .410 with runners in scoring position, and Manager Dick Williams said his records indicated that Gwynn had the best "RBI percentage" on the team.
In the playoffs, the Padres won the first NL pennant in its team history, defeating the Cubs in the National League Championship Series (NLCS) before losing the World Series to the Detroit Tigers. Gwynn batted .368 in the NLCS and his one-out single in the bottom of the ninth in Game 4 set up Steve Garvey's game-winning homer. Gwynn hit .263 (5-for-19) in the World Series, and flew out to Tigers left fielder Larry Herndon for the final out of the fall classic. During the offseason, Gwynn took less money to stay in San Diego by signing a six-year, $4.6 million contract with the Padres. Still, his new salary of more than $500,000 salary for 1985 represented a sizable raise over the $180,000 he would have received, or the $100,000 he had received in 1984.
Gwynn's physical appearance belied his athleticism. He stood 5 feet 11 inches (1.80 m), thick around the middle and thighs. Even at his athletic prime when he could really run, he weighed around 200 pounds (91 kg) and was considered pudgy. His roly-poly frame was a self-described "body by Betty Crocker", a reference to the food product brand. Towards the end of his breakout season in 1984, he conceded that his "extra weight hasn't helped me. My bat's slower than it has been all year." At the time, he attributed his weight gain to soft drinks: "It's killing me. It's always been a weakness. I've gotta cut down on the soda pop." Jim Murray of the Los Angeles Times opined in 1993 that Gwynn's "deceptive" looks and not "look[ing] the part" contributed to his regularly being overlooked in the voting for league MVP. Gwynn was sensitive about his weight. His knee injuries over his career prompted the Padres organization, media, and fans to question his weight, conditioning, and eating habits. However, he rejected the notion that his weight led to his injuries. Gwynn contended that his weight was only a topic when he was struggling, and not when he was performing. He also believed that the criticism stemmed mainly from his not fitting people's profile of what an athlete should look like. He said he had "a football player's body", with his father, grandfather and brothers also being big. Gwynn said his hitting style put a lot of torque on his knees. He started experiencing problems with his legs in 1986, when he suffered from shin splints. In his career, he had 13 operations, including eight involving his knees. Limited by injuries, he played over 135 games just once in his final 11 seasons. Gwynn used to run a great deal to keep in shape, but his last five years were mostly limited to working out in the gym. His actual weight is generally thought to have been under-reported during the latter part of his career, when it was officially listed between 215 and 220 pounds (98–100 kg).
Gwynn placed in the top 10 in voting for the National League MVP seven times in his career, including his only top-five finish in 1984, when he ended up third. In 2005, Sporting News ranked Gwynn No. 57 on the list of their 100 Greatest Baseball Players, and he was nominated as a finalist for the Major League Baseball All-Century Team. However, Gwynn's significance is muted by the non-traditional measurements in sabermetrics, which tends to favor power and the ability to get on base over batting average. As of 2014, his career 65.0 WAR ranked 34th among outfielders, and a few above him had not yet been voted into the Hall of Fame. While he had a first-rate on-base percentage of .388, he was one of only four players to hit .335 or more who did not have career .400 OBP. During his career, he finished in the top-10 in the NL in OBP 10 times. Gwynn's run production was another rap against him. He exceeded 90 RBIs in a season just once, when he had 119 in 1997, but he batted .349 in his career with runners in scoring position. "He was devastating with runners in scoring position. Impossible", former player Eric Davis said.