Grassi grew up in Montgomery County, Md. He was selected to the U.S. U-16 Men's National Team in 2004, moved up to the U-21 USMNT and in 2012 joined the senior USMNT, where he continues to play. Over his career he has competed in Junior Pan American Championships, Pan American Cups, and Pan American Games, in addition to FIH Hockey World League events and other tournaments. He is also a member of the Indoor USMNT.

Aside from his playing career, Grassi has been coaching field hockey for more than a decade. He is passionate about growing the game of field hockey for boys and girls. He has worked with several different club teams, in addition to collegiate teams and Junior USMNT. He runs Mustangs Field Hockey Club, for boys and girls youth hockey all the way up to adults.

Grassi joined the USA Field Hockey Board of Directors in 2019 as an interim Athlete Representative and was officially voted into the position as a full Athlete Representative later that year. He is also the Chair of the Athlete Advisory Committee. He currently lives in Washington D.C. and works as a mechanical engineer.