Undrafted, Lappin played collegiate hockey with Brown University of the ECAC. Upon completing his senior year in the 2015–16 season, with 33 points in just 31 games, Lappin was signed by the New Jersey Devils to a two-year entry-level deal on March 9, 2016.[2] He soon made his professional debut with the Devils AHL affiliate namesake, in Albany, appearing in 12 games for 7 points. In the post-season, Lappin continued his immediate contribution with 5 goals and 7 points in 11 games.[citation needed]
In his first full professional season in 2016–17, Lappin made his NHL debut for the Devils on October 28, 2016 against the Chicago Blackhawks.[3] He recorded his first NHL point with an assist one night later against the Tampa Bay Lightning.[4] On November 8, 2016, Lappin scored his first NHL goal in a 3–2 win against the Carolina Hurricanes.[5]
On July 1, 2019, Lappin left the Devils organization after 5 seasons to sign as a free agent on a one-year, two-way contract with the St. Louis Blues for the 2019–20 campaign.[6] Assigned to AHL affiliate, the San Antonio Rampage, for the duration of his contract with the Blues, Lappin produced just 5 goals and 11 points through 42 regular season games before the season was cancelled due to the COVID-19 pandemic.
As a free agent from the Blues and going un-signed leading into the pandemic delayed 2020–21 season, Lappin was signed to a one-year AHL contract with the Cleveland Monsters, affiliate of the Columbus Blue Jackets, on January 21, 2021.[7]
Lappin remained a free agent with the 2021–22 season underway, before opting to sign a professional try-out contract with the Lehigh Valley Phantoms of the AHL, affiliate to the Philadelphia Flyers, on November 23, 2021.[8]
Following the conclusion of his contract with the Phantoms, Lappin opted to halt his North American career in agreeing to a contract with Italian based club, HC Bolzano of the ICEHL, on July 22, 2022.[9]