As a freshman at Oregon State in 2016, Grenier played in 52 games with 49 starts, hitting .240 with 18 RBIs along with an on-base percentage of .342.[4] After the 2016 season, he played collegiate summer baseball with the Falmouth Commodores of the Cape Cod Baseball League.[5] In 2017, as a sophomore, he started all 62 of Oregon State's games, slashing .275/.393/.435 with five home runs, 37 RBIs, five doubles and six triples, earning him a spot on the Pac-12 First Team.[6] In 2018, his junior season, he was named the Pac-12 Defensive Player of the Year along with making the Pac-12 First Team for the second consecutive year.[7] He also won the Brooks Wallace Award.[8] He finished his junior year with a slash line of .319/.408/.462 with six home runs and 47 RBIs in 68 games,[9] helping the Beavers win the 2018 College World Series.[10] He was named to the College World Series All-Tournament Team.[11]
Grenier returned to Delmarva to begin the 2019 season. In July, he was promoted to the Frederick Keys.[15] Over 106 games between the two teams, he batted .244 with eight home runs and 43 RBIs.
Grenier retired from minor league baseball on April 5, 2023.[16]