(Updated Jan. 30, 2023)
Before Mark McGwire, who played for the Cardinals from 1997-2001, another Big Mac, Willie McCovey of the Giants, hit the longest home run seen in St. Louis.
Cardinals broadcaster Mike Shannon has said the longest home run he has witnessed was hit by McCovey on Sept. 4, 1966, at St. Louis.
Leading off the third inning against Cardinals starter Al Jackson, McCovey hit a changeup that landed in the upper deck above the scoreboard in right-center field at Busch Memorial Stadium. The ballpark opened four months earlier, in May 1966.
Shannon was playing right field for the Cardinals that day and “had a good look” at McCovey’s home run. Shannon said he later asked McCovey (who had 521 career home runs in the major leagues) whether it was the longest ball he’d hit. “I don’t know if it was the longest,” Shannon said McCovey replied, “but it was the hardest.” Boxscore
Cardinals catcher Tim McCarver told writer Roger Angell of The New Yorker magazine, “There’s no doubt in my mind about this. It was as if McCovey had hit it off a tee. It caromed off the upper part of the scoreboard and bounced back onto the field.”
The book “Baseball’s Ultimate Power: Ranking the All-Time Greatest Distance Home Run” had this description of McCovey’s St. Louis home run: “The ball was struck on a line drive trajectory that resulted in a 515-foot journey.”
The Cardinals’ 2005 Busch Stadium commemorative yearbook said many who witnessed McCovey’s blast will continue to regard it as the longest home run in that stadium’s history. “That may be the farthest hit anywhere,” Shannon said. “I’ll never forget it as long as I live.”
In the first inning, with the Cardinals using an infield shift against the left-handed slugger, McCovey bunted for a single. Asked by Jack Hanley of The San Mateo Times whether he got more pleasure from the bunt or from the mammoth home run, McCovey replied, “The more I think about it, the more I’m becoming convinced I get a bigger kick out of the bunt. It’s because, when you do the unexpected, the other fellow is completely surprised and it’s a trifle upsetting. The bunt can win a ballgame as much as a homer.”
(On Sept. 16, 1966, 12 days after his shot in St. Louis, McCovey hit a 505-foot home run off Mets starter Jack Fisher at San Francisco’s Candlestick Park. It’s the longest home run hit at that stadium, according to the San Francisco Examiner. Boxscore)
Until McGwire arrived, Pirates first baseman Willie Stargell came closest to challenging McCovey’s home run for longest hit at Busch Memorial Stadium. On July 4, 1979, Stargell hit a slider from reliever Darold Knowles 510 feet, above and to the right of the scoreboard. Boxscore
“That’s the longest home run I’ve ever seen hit in this ballpark by a left-hander,” Cardinals first baseman Keith Hernandez said to the Associated Press.
Said Stargell: “When I saw it go out, I saw (Knowles) flinging something like his cap. He was disgusted. It was a ball that Darold, I’m sure, got in an area he didn’t want. It was a nice, easy swing. I had no idea it was going that far.”
Nineteen years after Stargell’s shot, McGwire hit what officially is called the longest home run at Busch Memorial Stadium. The 545-foot home run on May 16, 1998, against the Marlins’ Livan Hernandez hit the St. Louis Post-Dispatch sign in center field. For the remainder of the season, a giant Band-Aid marked the spot where the ball dented the sign. Boxscore
“It’s the best ball I’ve ever hit,” McGwire said. “I don’t think I can hit one better than that.”
RetroSimba 
I saw Willie McCovey hit a homerun in Atlanta Fulton County Stadium back in ’65 or ’66 I think. It was the longest hit ball I have ever seen … into the upper deck of center field. The stadium was packed as it always was back in those days when the Giants or Dodgers came to town. McCovey hit the ball and there was a resounding unified “Ooooooooo” that reverberated through the stands as all watched the towering homerun fly well into the upper deck of center field … a mile high and two miles long it seemed. The Atlanta faithful were astounded and rose to their feet to applaud and cheer for McCovey’s extraordinary blast. I will never forget it.
Thank you for sharing your vivid description of the McCovey home run. Awesome. Wish I’d have seen him hit one in person. Never got the chance. And to think he did all that without performance-enhancing drugs!
First upper deck HR in Atlanta Stadium didn’t happen until 1969, by a Cub named Smith.
I saw McCovey’s in person and I thought I was 12 or 13 and that would have been ’65 or ’66 but I could have been 15 or 16 if your statement is true.
McCovey’s best homer was his first one hit against the Dodgers on Willie’s first ever Giant at bat as a pinch hitter. It was a walkoff sometime in his first pro year.
I saw it, on that hot Sunday afternoon. Me and my buddy, both 13 years old, had ‘snuck’ into the field boxes, just behind the 3rd base dugout. I remember, to this day, how far it went and where it landed. I’ve seen a lot of home runs hit since then, but never one that far!
Thanks for sharing the first-hand insights, Brian. It’s very special that you got to see that.
I was 11 or 12 when I saw him hit one in San Diego while he was playing with the Padres near the end of his career. I swear that it either flew out of SD Stadium/Jack Murphy/Qualcomm or hit right something in the outermost area and bounced out into the parking lot. The ball was “never” seen again. I remember asking my Grandpa: “Where’d it go?” “Do you think it went out of the Stadium?”. I wonder if it really did go out. I wonder if anyone else happened to be there that day in ’74-’76 (don’t remember exactly). It was definitely a moment that took your breath away as it was just *crushed*…I’ll bet the ball was lopsided after that! ;)
Thank you for sharing that experience. Getting to witness a long home run from Willie McCovey is a delight.
Willie Mac my idol. Saw him hit many at “the stick”. Thanks for the memories.
Willie McCovey is a good choice for a favorite player.
Although I only got to watch him on Game of the Week, he was absolutely electrifying. Requiescet in pace, Mac.