"Baseball is 90 percent mental” has been cited in print since at least 1974. Kansas City Royals outfielder Jim Wohlford was cited in April 1974 for saying::
“Ninety per cent of this game is half mental.”
“Ninety per cent of this game is half mental” was credited to baseball manager Danny Ozark (1923-2009) in April 1977. “Ninety percent of this game is half mental” was also credited to baseball manager Yogi Berra (1925-2015) in January 1979. Berra usually receives credit for the line, but Wohlford said it first. “Baseball is ninety percent mental, and the other half is physical” is a popular variation of the line.
[This entry includes research assistance from the Quote Investigator.]
Wikipedia: Jim Wohlford
James Eugene Wohlford (born February 28, 1951 in Visalia, California) is a retired former professional baseball player. outfielder. In his 15-year Major League Baseball career, he played for the Kansas City Royals (1972–76), the Milwaukee Brewers (1977–79), San Francisco Giants (1980–82), and Montreal Expos (1983–86), primarily as an outfielder.
Wikipedia: Danny Ozark
Daniel Leonard Ozark, born Daniel Leonard Orzechowski (November 24, 1923 — May 7, 2009) was an American coach and manager in Major League Baseball. As manager of the Philadelphia Phillies (1973 through August 31, 1979), Ozark led the Phils to three consecutive National League East Division championships (1976-77-78), but each year his team fell in the National League Championship Series.
Wikiquote: Yogi Berra
Lawrence Peter “Yogi” Berra (May 12, 1925-September 22, 2015) is a former American baseball player, manager and member of Major League Baseball’s Hall of Fame, noted for his malapropisms.
Ninety percent of this game is half-mental.
. What Time Is It? You Mean Now?: Advice for Life from the Zennest Master of Them All, Simon and Schuster, 2003, ISBN 0743244532, p. 45. This version is also attributed to Philadelphia Philles manager Danny Ozark.
. Variant: Ninety percent of this game is mental, and the other half is physical.
4 April 1974, Maryville (MO) Daily Forum, “‘Needlee’ Wohlford May Have Last Laugh” by Steve Cameron, pg. 9, cols. 1-2:
Kansas City—Jim Wohlford is the kind of guy who was just made to take abuse.
He’s the kind of guy who manages, quite innocently, to always say the kind of thing that will haunt him ("Ninety per cent of this game is half mental...").
19 July 1974, New Castle (PA) News, “‘Babs’ Tanner helps make husband successful pilot” by Pete Mollica, pg. 11, col. 2:
“Baseball is 90 percent mental.”
(Chuck Tanner, manager of the Chicago White Sox.—ed.)
31 January 1976, Maryville (MO) Daily Forum, “Notes and Quotes from Leiker” by Ken Leiker, pg. 2, col. 2:
Walker and Hansen, it appears, may be ready to challenge the Royals’ Jim Wohlford, who once said, “Ninety per cent of this game is half mental.”
19 April 1977, Des Moines (IA) Register, Maury White column, pg. 1S, col. 1:
“His limitations are limitless,” is an oft-repeated Ozarkism.
“I’m leaving no leaf unturned,” is another.
“Ninety per cent of this game is half mental,” say Danny (Ozark—ed.), equalling The Best of Yogi Berra on any given day.
Google News Archive
26 April 1977, St. Petersburg (FL) Times, Sports Editor Hubert Mizell, pg. 1C, col. 1:
DANNY OZARK, the Phillies’ manager, is a nice man, but he misuses the language to a point that even the late Casey Stengel would’ve become confused. Ozark has said “One reason we were good last year is that our pitching staff led the league in walks not given up.” And, “Ninety per cent of this game is half-mental.”
9 July 1977, Midland (TX) Reporter-Telegram, “Trouble Rangers pass milestone” by Ted Battles, pg. 1B, col. 4:
“Baseball is 90 per cent mental and I just feel better playing at home.”
(San Francisco Giant player Bill Madlock.—ed.)
24 September 1977, The Saturday Herald and Leader (Lexington, KY), “The Last Word,” pg. B-2, col. 2:
Milwaukke Brewers’ outfielder Jim Wohlford, commenting on baseball:
“Ninety percent of this game is half mental.”
2 October 1977, Boston (MA) Herald American, pg. D11, col. 2:
JIM WOHLFORD of the Milwaukee Brewers, a throw-back to Yogi Berra, sums up big league baseball: “Ninety percent of the game is half mental.”
Washington (DC) Post
‘Playing Harder’ Gets Terps No. 6 Rankings
By Betty Cuniberti October 18, 1978
When Jim Wohlford played for the Kansas City Royals, he said of baseball, “Half of this game is 90 percent mental.”
28 January 1979, Aberdeen (SD) American News, “Patrons read, but don’t scribble” by Dick Young (New York Daily News), pg. 11, col. 1:
“Ninety percent of this game is half mental.”—Yogi Berra.
6 February 1979, Marietta (GA) Daily Journal, “In Bitter Contract Dispute: Both Horner, Braves Could Be Losers” by Mick Walsh, pg. 1B, col. 1:
Unlike Yogi Berra who insists that 50 per cent of baseball is 90 per cent mental, it’s my considered opinion that baseball is 90 per cent players, 10 per cent agents.
1 August 1979, Evansville (IN) Courier, “Sidelines” by Bill Fluty, pg. 23, col. 2:
Manager Danny Ozark, on baseball: Half this game is 90 percent mental.
(From Bert Sugar’s The Book of Sports Quotes.—ed.)
26 October 1979, Rockford (IL) Register Star, “They said it all before...much better, too” by Charlie Rayman, pg. D3, col. 1:
Danny Ozark—“Half this game is 90 percent mental.”
19 June 1980, The Morning Union (Springfield, MA), “Reborn A’s raising eyebrows” by Gerry Flynn, pg. 35, col. 5:
“It’s all mental,” pitcher Matt Keough characterized the turnaround. “You know the old Yogi Berra joke about baseball being 90% mental and the other half physical? Well, it’s the truth even though Yogi screwed up the breakdown.”
Los Angeles (CA) Times
Yogi Berra dies at 90; greatest quotes
By HOUSTON MITCHELL
SEPTEMBER 22, 2015, 11:40 PM
“Baseball is 90% mental. The other half is physical.”