Pastrnak's NHL debut came on 24 November 2014, and it didn't take long for him to make an impact. Although the Bruins lost in overtime to the Pittsburgh Penguins, Pastrnak showed flashes of his potential, logging 7:53 of ice time. On 10 January 2015, he scored his first two NHL goals in a Bruins' road win over the Philadelphia Flyers. His first goal came just 1:10 into the game, and his second goal sealed the 3-1 victory for the Bruins.

Pastrnak continued to impress in his rookie season, becoming the youngest Bruins' player ever to score an overtime, game-winning goal in regular-season play on 29 March 2015, in a 2-1 win over the Carolina Hurricanes. At the end of the season, he was one of just two players selected in the 2014 NHL Entry Draft to play in more than 40 NHL games during the 2014-15 season.

In the 2015-16 season, Pastrnak continued to make history, becoming the youngest Bruins' player ever to score a penalty shot goal in a regular-season home game against the Penguins. He also earned the nickname "Pasta" from Bruins' fans.

Pastrnak's breakout year came in the 2016-17 season when he recorded 34 goals and 70 points to lead the Bruins in scoring. He also scored two goals and four points for the team during the 2017 playoffs. In the offseason, Pastrnak signed a six-year, $40 million contract extension with the Bruins.

During the 2017-18 season, Pastrnak recorded his first NHL hat-trick during a come-from-behind 6-4 Bruins' road win over the Carolina Hurricanes. He also recorded another hat-trick along with three assists in a 7-3 home win over the Toronto Maple Leafs in game two of the Bruins' First Round series during the 2018 playoffs, becoming the youngest player in NHL history to score six points in a single playoff game.

Pastrnak's skills were on full display during the 2019 NHL All-Star Skills Competition, where he won the Accuracy Shooting competition, hitting five targets in 11.309 seconds. He also became the all-time leader for most goals in Boston Bruins history before the age of 23, passing Barry Pederson, in a game against the Philadelphia Flyers.

On 14 October 2019, Pastrnak scored all four goals in a 4-2 win over the Anaheim Ducks, becoming the third player in Bruins' history to accomplish the feat. He also became the first player in Bruins history to record three 30+ goal seasons before the age of 23, passing the two of Bobby Orr, Cam Neely, and Barry Pederson.

David Pastrnak's impressive skills and infectious personality have made him a fan favorite in Boston and around the NHL. From his early days as a rookie to his recent record-setting performances, Pastrnak has proven himself to be one of the most exciting players in the game today. As he continues to develop his skills and lead the Bruins to new heights, fans can look forward to many more memorable plays from this talented young star.