A native of Lunenburg, Jefferies played his early junior career with the local Islanders Hockey Club before he began attending The Gunnery in 2018. He led the prep team in scoring two years in a row and was able to earn a scholarship to Merrimack as an 18 year old. Despite the end of the 2020 season being cancelled due to the coronavirus pandemic, the New York Islanders had seen enough from Jefferies to select him in the 4th round of the NHL draft.[2]
During the abbreviated Covid season, Jefferies played sparingly. While he only appeared in 12 games with the Warriors, he still was nearly a point per game player and finished 3rd on the team in scoring. His production slumped a bit during his sophomore season and though he did help the team post a winning record, the Warriors ended the year as a pedestrian club. It wasn't until his junior season that Jefferies began to demonstrate the talent that he had possessed before the pandemic. While nearly doubling his point total from the previous season, Jefferies led the Merrimack offense and was a major reason for the program reaching the NCAA tournament for just the second time at the D-I level.[3] While their stay wasn't long, the stellar season earned Jefferies a spot on the All-American team and had Islander fans salivating over the prospect of him in a New York jersey.[4] Those hopes would have to wait, as Jefferies announced that he was returning to Merrimack for his senior season.[5]