McGwire was just 23 years old when he set the rookie record 30 years ago, two years younger than Judge.
"I am swinging at the right pitches, getting into good counts and swinging at the right pitches", Judge said.
His 50th homer came off the pitching of Trevor Cahill, with the ball flying into the left field seats.
McGwire had clubbed 49 for Oakland in 1987.
New York Yankees outfielder Aaron Judge etched his name in Major League Baseball record books once again on Monday breaking Mark McGwire's rookie record with his 49th and 50th home runs of the season.
Judge has heated up lately after enduring a mostly hard second half. McGwire hit.289 and struck out 131 times when he set the record.
Judge has now hit 13 home runs in September after a terrible month of August which saw him only hit three with a paltry.185 batting average. He connected twice in Yankee Stadium, giving him 50 home runs on the season.
Only Miami Marlins outfielder Giancarlo Stanton has hit more home runs than Judge this season - he has 57 as the regular season comes towards a close.
The Royals (76-79) are six games behind the Minnesota Twins in the race for the second American League wild-card spot.