After redshirting in 2010 with the Sooners, Grissom played as a reserve defensive tackle in his 2011 and 2012, picking up 9 total tackles and one tackle for a loss. In 2013, Grissom saw his first career start come against ULM Warhawks in which he recorded two tackles. His first interception came on October 12 against the Texas Longhorns, which he returned 54 yards for a touchdown. In the Sugar Bowl against Alabama, Grissom recorded 2.5 sacks, broke up a pass, and recovered two fumbles. He returned one of these fumbles for an eight-yard touchdown and had a total of 34 return yards. As a senior at Oklahoma in 2014, Grissom was named an honorable mention for the All-Big 12 after recording 39 tackles and 3.5 sacks on the year. Over his four seasons at Oklahoma, he registered 88 tackles, 8 sacks, 2 forced fumbles, 3 fumble recoveries and 2 interceptions while playing in 39 games with 12 starts.
Professional career
On November 10, 2014, it was announced that Grissom had accepted his invitation to play in the 2015 Senior Bowl. On January 24, 2015, Grissom appeared in the Senior Bowl for Jacksonville Jaguars'Gus Bradley's South team and assisted on three tackles during their 34-13 loss. He was one of 33 collegiate linebackers to attend the NFL Scouting Combine in Indianapolis, Indiana. Grissom performed all of the combine drills and finished fifth among all linebackers in the vertical jump, sixth in the bench press, tied for 14th in the broad jump and three-cone drill, and 20th in the 40-yard dash. On March 11, 2015, he participated at Oklahoma's pro day and opted to perform nearly all of the combine drills again. Unfortunately, he was unable to have a better performance, but was able to impress team representatives and scouts in positional drills. He also showed his versatility and athleticism by also taking part in tight end drills at Oklahoma's pro day. New England Patriots' head coach Bill Belichick stated that Grissom's tight end drills were, "As good as we saw all year." At the conclusion of the pre-draft process, Grissom was projected to be a fifth or sixth round pick by NFL draft experts and scouts. he was ranked the 18th best outside linebacker prospect in the draft by NFLDraftScout.com.
New England Patriots
The New England Patriots selected Grissom in the third round of the 2015 NFL Draft. He was the 14th defensive end selected in 2015. Belichick also stated Grissom could possibly play interior defensive line positions.
2015
On May 9, 2015, the New England Patriots signed Grissom to a four-year, $2.89 million contract that included a signing bonus of $581,948. Throughout training camp, he competed against Michael Buchanan, Jake Bequette, Zach Moore, and Xzavier Dickson. He was named the third right defensive end on the Patriots' depth chart to begin the regular season, behind Rob Ninkovich and Jabaal Sheard. He made his professional regular season debut in the New England Patriots' season-opener against the Pittsburgh Steelers and assisted on one tackle during their 28-21 victory. He made his first career tackle with teammate Alan Branch on Steelers' running backDeAngelo Williams after he ran for a six-yard gain in the first quarter. On October 29, 2015, Grissom recorded a season-high two combined tackles as the Patriots routed the Miami Dolphins 36-7. On January 1, 2016, Grissom made one solo tackle and his first career sack on Dolphins' quarterback Ryan Tannehill in New England's 20-10 loss to the Miami Dolphins. He was mainly used on special teams and as a reserve on defense his rookie season. Grissom finished the season with six combined tackles and one sack in 14 games. The New England Patriots finished the season atop the AFC East with a 12-4 record. Grissom was inactive throughout the playoffs as the Patriots went on to lose in the AFC Championship to the eventual Super Bowl 50 Champions the Denver Broncos.
2016
He competed with Chris Long, Trey Flowers, and Frank Kearse throughout training camp for a backup defensive end role. On September 3, 2016, he was released by the Patriots as part of final roster cuts and was signed to the practice squad the next day. He was promoted to the active roster on October 15, 2016. On November 27, 2016, Grissom assisted on two tackles during a 22-17 victory as the New York Jets. On January 14, 2017, he appeared in his first career playoff game during a 34-16 victory over the Houston Texans in the AFC Divisional round.
2017
Grissom was released by the Patriots on September 2, 2017 and was signed to the practice squad the next day. He was promoted to the active roster on September 22, 2017. He was released by the Patriots on November 9, 2017 and was re-signed to the practice squad. He was promoted back to the active roster on December 2, 2017.
2018
On November 6, 2018, Grissom was waived by the Patriots.
