He played college football for the NC State Wolfpack and was selected by the Tampa Bay Buccaneers in the third round of the 2013 NFL draft.

He was also a member of the Chicago Bears, Arizona Cardinals, Oakland Raiders, Jacksonville Jaguars, New York Giants, and Miami Dolphins.

Glennon led his team to an undefeated 15–0 record and the Virginia Division 6 AAA State championship title when the Bulldogs defeated the Woodbridge High School Vikings 42–14.

[16] On April 26, 2013, Glennon was selected by the Tampa Bay Buccaneers in the third round (73rd overall pick) of the 2013 NFL draft.

He earned his first win as an NFL starter the next week against the Miami Dolphins, though he had just 139 passing yards and an interception.

[25] Glennon was named to the Pro Football Writer's Association's All-Rookie Team for his strong first year performance, becoming the second Buccaneers quarterback to receive this award, joining Doug Williams (1978).

[27] New head coach Lovie Smith quickly announced that newly signed journeyman Josh McCown would be the starting quarterback for the upcoming 2014 season, leaving Glennon's future in Tampa Bay in question.

That same day, Rick Stroud of the Tampa Bay Times reported that the Bucs turned down at least six trade offers for Glennon during the 2014 NFL draft.

[29] Glennon made his 2014 season debut Week 3 on Thursday Night Football against the Atlanta Falcons.

[32] However, the Bucs lost their next four games (two in overtime), and Smith brought McCown back as starter in Week 10.

After Winston suffered a knee injury midway through the fourth quarter of the Week 9 game against the Falcons, Glennon entered and completed 10 of 11 passes for 75 yards and a touchdown.

[39] On September 10, 2017, in his Bears regular season debut, Glennon finished 26-of-40 for 213 yards and a touchdown in a 23–17 home loss to the Falcons at Soldier Field.

In the next game, he threw two interceptions and lost two fumbles in a Week 4 Thursday Night Football loss to the Green Bay Packers.

[41] On October 2, seven days before the Week 5 game against the Minnesota Vikings, it was announced that Glennon had been benched in favor of rookie first-round pick Mitchell Trubisky.

[44] On February 28, 2018, the Bears' general manager Ryan Pace announced that Glennon would be released at the start of the new league year.

[58] In Week 14 against the Tennessee Titans, Glennon threw for 85 yards and an interception before being benched for Gardner Minshew during the 31–10 loss.

[61][62] In Week 13 against the Dolphins, Glennon got his first start of the season due to Daniel Jones being injured.

[66] He was elevated to the Dolphins' active roster for the team's week 18 game against the New York Jets on January 8 to backup rookie Skylar Thompson after both starter Tua Tagovailoa and backup quarterback Teddy Bridgewater dealing with injuries.