Johnson, who returns as defending champion after claiming the Green Jacket in record-breaking fashion in November, will begin his title defence at 3.30pm (BST) on Thursday alongside Westwood - making his 20th Masters appearance - and last year's US Amateur champion Tyler Strafaci.
Four-time major champion Brooks Koepka, making his first competitive appearance since undergoing knee surgery last month, is another of the morning starters and goes out two groups earlier at 3.06pm BST with Viktor Hovland and two-time Masters winner Bubba Watson.
The two marquee threeballs have also been confirmed as the first two Featured Groups on Thursday, with live coverage beginning at 2pm on Sky Sports' dedicated Masters channel, with Jon Rahm and Rory McIlroy also part of the morning wave.
McIlroy - who has another opportunity to complete golf's career Grand Slam this week - plays alongside Rahm and world No 6 Xander Schauffele at 3.42pm on Thursday, with 2018 champion Patrick Reed in the next group out with Paul Casey and Daniel Berger.
Jordan Spieth, fresh off the back of ending his three-and-a-half-year run without a win at the Valero Texas Open on Sunday, is in the final group out on the opening day with PGA Champion Collin Morikawa and Masters 2020 runner-up Cameron Smith.
World No 2 Justin Thomas faces a late start alongside Tony Finau and Louis Oosthuizen, while US Open champion Bryson DeChambeau is also part of the afternoon wave and is in a threeball with Max Homa and 2013 winner Adam Scott.
1506 Bubba Watson, Brooks Koepka, Viktor Hovland
1530 Dustin Johnson, Lee Westwood, Tyler Strafaci
1542 Xander Schauffele, Jon Rahm, Rory McIlroy
1554 Patrick Reed, Daniel Berger, Paul Casey
1812 Phil Mickelson, Tommy Fleetwood, Scottie Scheffler
1836 Adam Scott, Bryson DeChambeau, Max Homa
1848 Tony Finau, Louis Oosthuizen, Justin Thomas
1900 Jordan Spieth, Cameron Smith, Collin Morikawa
Watch The Masters this week live on Sky Sports, with all four rounds exclusively live on Sky Sports' Masters channel. Live coverage beings with Featured Groups from 2pm on Thursday April 8.