[1] Following the signing, Weegar participated in the 2014 Prospect Tournament alongside fellow Panthers rookies Logan Shaw and Vincent Trocheck.

[2] After participating in the Panthers' five preseason games, Weegar was re-assigned to their American Hockey League (AHL) affiliate, the San Antonio Rampage, to start the 2014–15 season.

[17] Entering the final year of his entry-level contract, Weegar returned to the Panthers' new AHL affiliate, the Springfield Thunderbirds, for the 2016–17 season.

[20][21] On October 26, Weegar scored the game-winning goal in overtime to add to his team-leading four points to lead the Thunderbirds to their first home victory.

[30] He eventually made his season debut on October 20 and subsequently scored his first career NHL goal that night against the Pittsburgh Penguins.

During a 3–2 road win over the New Jersey Devils in November, Boughner moved Weegar up to the second defensive pairing with Mike Matheson and he played 20 minutes of ice time.

[36] Weegar improved offensively for the Panthers during the 2018–19 season and set new career highs in goals, assists, and games played while also averaging a career-best 16:58 of ice time.

[37] Although he missed the majority of training camp due to a shoulder injury,[38] Weegar was named to the Panthers opening night roster for the second consecutive season.

[40] However, after the NHL All-Star break, Weegar began consistently playing alongside Keith Yandle on the Panthers second defensive pairing.

When new head coach Joel Quenneville put Weegar and Aaron Ekblad on their top unit, they combined for 5 points, 14 shots on goal and a plus-6 rating over three games.

When he returned to the lineup on February 4, 2020, he recorded one shot, two hits, one block and one takeaway in 13:29 of ice time alongside Keith Yandle.

Although he sat out for their exhibition loss to the Tampa Bay Lightning, Weegar and Ekblad were expected to return to the lineup for Game 1 of the Stanley Cup Qualifiers.

[51][52] Following the Flames not qualifying for the 2023 Stanley Cup playoffs, Weegar accepted his first invitation to join Team Canada for the 2023 IIHF World Championship.

Recording three goals and eight assists in ten games, he led the team and all defencemen in the tournament in points on the way to winning a gold medal.