2021 6 Hours of The Glen

[3] The race marked IMSA's return to Watkins Glen after the previous year's edition of the event was canceled as a result of the COVID-19 pandemic.

[4] On June 18, 2021, IMSA released the latest technical bulletin outlining Balance of Performance for the event.

[5] In DPi, The Acura and Cadillac received slight fuel capacity reductions at one and two liters respectively, while the Mazda featured a 3.6 kW horsepower increase.

The two GT classes saw minor adjustments, with the GTLM-class Porsche receiving a 10 kg weight reduction, while the Aston Martin received a 7.1 kW horsepower decrease and a two-liter fuel capacity decrease after winning the previous round at Belle Isle.

[6] Nicky Catsburg, Antonio García, and Jordan Taylor led the GTLM Drivers' Championship with 697 points, 41 points ahead of Cooper MacNeil followed by John Edwards, Augusto Farfus, and Jesse Krohn in third.

[6] With 920 points, the GTD Drivers' Championship was led by Bill Auberlen and Robby Foley, ahead of Roman De Angelis and Ross Gunn.

[7] DPi featured the expected addition of the Ally Cadillac Racing #48 alongside the six full-season entries in the class.

In LMP2, United Autosports returned as part of their Michelin Endurance Cup schedule, while James French joined Tower Motorsport By Starworks for the first time in 2021.

[9] LMP3 featured a series of additions, including Marco Andretti's addition to the Andretti Autosport entry, United Autosports' IMSA LMP3 debut, and the long-awaited first IMSA SportsCar Championship appearance for Dawson Racing.

[8] In GTD, CarBahn with Peregrine Racing added another event to their Sprint Cup schedule,[8] and NTE Sport returned after most recently appearing at Daytona in January.

[11] Wright Motorsports also announced prior to the event that Trent Hindman would replace Ryan Hardwick as Patrick Long's co-driver for the remainder of the season.

[12] The first practice session took place at 4:20 pm ET on Friday and ended with Oliver Pla topping the charts for Meyer Shank Racing with Curb-Agajanian, with a lap time of 1:31.056, ahead of the No.

3 Corvette Racing entry was second fastest and Jesse Krohn rounded out the top three.

79 WeatherTech Racing Porsche 911 RSR-19 suffered suspension damage after going airborne at the Inner Loop causing debris to be scattered onto the track.

[16] The second and final practice session took place at 8:00 am ET on Saturday and ended with Dane Cameron topping the charts for Meyer Shank Racing with Curb-Agajanian, with a lap time of 1:30.451.

3 Corvette Racing entry was second fastest and John Edwards rounded out the top three.

11 WIN Autosport Oreca 07's brakes at turn eight and hit the wall before collecting Zacharie Robichon's No.

[25] Jack Hawksworth set the fastest time in the GTD points paying session driving the No.

[35] As a result of winning the race, Merrill, Nunez, and Thomas advanced from fourth to second in the LMP2 Drivers' Championship.

[35] With a total of 1090 points, Robinson's victory allowed him to increase his advantage over Bennett and Braun in the LMP3 Drivers' Standings.

[35] By finishing third place, De Phillippi, Eng, and Spengler advanced from fifth to third in the GTLM Drivers' Championship.

[35] With a total of 1296 points, Auberlen and Foley's victory allowed them to increase their advantage over De Angelis and Gunn in the GTD Drivers' Championship.