Thielen holds several NFL records, including eight straight games over 100+ yards receiving, and 74 receptions in the first half of a season.

[1] Born in Detroit Lakes, Minnesota, to Pete and Jayne Thielen, Adam Thielen with his cousins grew up rooting for the Vikings and idolized Pro Football Hall of Fame wide receivers Cris Carter and Randy Moss, and would wear their Vikings jerseys while practicing in his back yard as a child.

[2][3] He attended Detroit Lakes High School, where he participated in four sports (football, basketball, baseball and golf).

[5] After redshirting the 2008 season at Minnesota State University, Mankato, Thielen played in nine games and caught 21 passes for 252 yards and a touchdown as a freshman in 2009.

As a sophomore in 2010, he started in all 11 games and was named Minnesota State's Offensive Player of the Year in addition to the All-Northern Sun Intercollegiate Conference (NSIC) South Division second-team after he led the team with 41 receptions for 686 yards and six touchdowns.

During the 2012 season, Thielen became the main target for Minnesota State's passing attack and was again the team's starting kick and punt returner, helping the Mavericks to a 13–1 record, as they advanced to the NCAA Semi-finals.

For his efforts, Thielen was named to the NSIC South Division All-NSIC first-team, the Daktronics Super Region No.

[10] After going undrafted in the 2013 NFL draft, Thielen attended a rookie tryout and evaluation session at Winter Park with the Minnesota Vikings.

[13] On August 31, 2013, the Minnesota Vikings waived Thielen as part of their final roster cuts (along with 18 others), but he was signed to the practice squad the next day.

After a strong preseason performance in both offense and special teams, Thielen earned a spot on the 53-man roster to start the 2014 campaign.

[15] Thielen made his NFL debut in the season opener in a road game against the St. Louis Rams on September 7.

[16] In Week 5, he caught his first NFL pass in the Vikings' 42–10 road loss against the Green Bay Packers, finishing the game with four receptions for 57 yards that led the team in receiving.

[19] In Week 17 against the Chicago Bears, Thielen scored his first receiving touchdown on a 44-yard pass from quarterback Teddy Bridgewater, after which he promptly gave the ball to his wife (then fiancée) Caitlin.

[23] In Week 4, Thielen hauled in a career-high six passes for 70 yards in a road game at the eventual Super Bowl champions Denver Broncos.

[27] With Stefon Diggs sidelined due to a groin injury, Thielen received the starting nod in his place for Week 5.

[28] In the Vikings' loss to the Washington Redskins in Week 10, Thielen caught his second touchdown of the season on a 3-yard toss from Sam Bradford and also carried the ball once for 11 yards.

[29] Thielen had a career day on December 24, 2016, in a Week 16 loss to the Packers with a 202-yard, two-touchdown performance at Lambeau Field.

[31][32] After having entered the 2016 season as an underdog, Thielen ended up leading the Vikings with 967 receiving yards, while also adding a career-high 69 catches and five touchdowns.

[35] Thielen dropped only two passes on 92 targets according to Sporting Charts, and Pro Football Focus ranked him as the 18th best receiver in the league.

[39] In Week 8, Thielen scored his first touchdown of the season against the Cleveland Browns at Twickenham Stadium, London from quarterback Case Keenum.

The following week against the Los Angeles Rams, Thielen finished the game with six catches for 123 yards, including a 65-yard touchdown reception.

[41] During the Week 12 Thanksgiving matchup against division rival, Detroit Lions, Thielen had eight receptions for 89 yards in the Vikings 30–23 victory.

With this fourth consecutive game with at least 100 receiving yards to start the season, he joined Randy Moss and Isaac Bruce as the only players in the Super Bowl era to accomplish the feat.

[71] In Week 5 against the Seattle Seahawks on Sunday Night Football, Thielen recorded nine catches for 80 yards and two touchdowns during the 27–26 loss.