As an Angel, he preserved Clyde Wright's no-hitter against the Athletics in the seventh inning by catching a Reggie Jackson fly ball 400 feet from straightaway center field, just in front of the wall (July 3, 1970).
As a Dodger, he hit a pinch-two run home run in Game Four of the 1981 World Series against the New York Yankees, the home run rallying the Dodgers from a 6–3 deficit to win 8–7. The victory also enabled the Dodgers to tie the Series at two games each; they won the next two games to win it all.
He pulled off a number of infamous pranks during his playing days, including placing a soggy brownie inside Steve Garvey's first base mitt, setting teammates' cleats on fire (known as "hot-footing"), cutting out the crotch area of Rick Sutcliffe's underwear, locking Dodger manager Tommy Lasorda in his office during spring training, once dressing up as a groundskeeper and sweeping the Dodger Stadium infield in between innings and then hitting homers the next, nailing teammates' cleats to the floor, and replacing the celebrity photos in manager Lasorda's office with pictures of himself, Jerry Reuss and Don Stanhouse. One time, during pre-game warm ups, he climbed atop the Dodger dugout and, in full game uniform, walked through the field boxes at Dodger Stadium to the concession stand and got a hot dog. He also once dressed up in Lasorda's uniform (with padding underneath) and ran out to the mound to talk to the pitcher while carrying Lasorda's book and a can of Slim Fast.
As a baseball announcer, he once covered a microphone with a scent of stale eggs then proceeded to interview Dave Stewart, Mickey Hatcher and other players.
Many of the pranks, along with other aspects of his career, are described in the books he co-authored with sports columnist Rick Talley – Temporary Insanity, Over the Edge, and Some of My Best Friends Are Crazy.
Johnstone appeared in the movie Naked Gun as a member of the Seattle Mariners in a game against the California Angels. Johnstone, who was a left-handed hitter throughout his career, bats right-handed in the movie.