The Washington Redskins selected Nicholson in the fourth round of the 2017 NFL Draft. On May 11, 2017, Nicholson signed a four-year, 3 million contract. Throughout training camp, he competed for the job as a backup safety against Will Blackmon, Deshazor Everett, Earl Wolff, Josh Evans, Stefan McClure, and Fish Smithson. Head coach Jay Gruden named him the backup strong safety, behind Deshazor Everett, to begin the regular season after the original starting strong safety, Su'a Cravens, announced his decision to retire a week before their season-opener. in 2017. He made his professional regular season debut in the Washington Redskins' season-opening 30–17 loss to the Philadelphia Eagles. On September 17, 2017, Nicholson earned his first career start and made two solo tackles in the Redskins' 27–20 victory at the Los Angeles Rams in Week 2. Defensive coordinatorGreg Manusky opted to use Nicholson in nickel packages and Everett in base formations. In Week 3, Nicholson recorded a solo tackle, a pass deflection, and made his first career interception off a pass by Derek Carr during a 27–10 victory against the Oakland Raiders. On October 15, 2017, he recorded a season-high six combined tackles and broke up a pass, but left in the second quarter of the Redskins' 26–24 win against the San Francisco 49er after sustaining a concussion. His concussion occurred in the last 45 seconds of the half after Nicholson attempted to tackle wide receiverPierre Garcon, but lowered shoulder and took a blow directly to his head that knocked off his helmet and rendered him unconscious. His concussion sidelined him for the next two games. On November 18, 2017, he made four solo tackles before leaving in the third quarter of the Redskins' 20–10 victory against the New York Giants after receiving another concussion. On December 19, 2017, Nicholson was placed on injured reserve. He finished his rookie season with 24 combined tackles, two pass deflections, and an interception in eight games and six starts.
2018
Nicholson entered the 2018 season as the Redskins starting free safety alongside strong safety D. J. Swearinger. He started the first seven games before losing the starting job in Week 9 after the Redskins traded for Ha-Ha Clinton-Dix. He was placed on the reserve/NFI list on December 19, 2018 following an assault and battery arrest.
2019
In week 2 against the Dallas Cowboys, Nicholson recorded his first interception of the season off Dak Prescott and returned it 23 yards as the Redskins lost 31-21. In week 5 against the New England Patriots, Nicholson intercepted Tom Brady once in the 33-7 loss. He was placed on injured reserve on December 27, 2019. He finished the season with 62 tackles, four passes defensed, and two interceptions through 13 starts. He was waived on March 23, 2020, with a failed physical designation.
Personal life
On December 18, 2018, Nicholson was arrested and charged with public swearing, intoxication, and misdemeanor assault in Ashburn, Virginia. The charges were dropped in May 2019. On November 14, 2019, it was reported that the Loudoun County Sheriff’s Office was investigating Nicholson regarding the death of 21 year-old Julia Crabbe. On the night of Crabbe's death, Nicholson and another man were identified as the ones who drove Crabbe to a hospital. Crabbe's cause of death was linked to drug overdose, with Nicholson not being officially charged.
