A baby-faced lefthanded hitter, Joyner replaced Rod Carew as the California Angels' first baseman in 1986. He became the first rookie to start an All-Star Game since fan balloting returned in 1970 and finished a close second to Jose Canseco in the AL Rookie of the Year balloting. After a brilliant first half, he struggled during the second half and missed four games of the League Championship Series against the Boston Red Sox with a staph infection.

In 1987 Joyner was the team's MVP as he clubbed 34 homers, a franchise record for first basemen, and became the ninth player in ML history with back-to-back 100-RBI seasons as a rookie and sophomore. His power declined in 1988 (13 dingers) as the ball was deadened, but he still managed 85 RBI, though he would never reach the power numbers he had in 1987. After a slow start in 1989 he finished with 16 taters and 79 RBIs while batting .282.

After compiling two more similar seasons for the Angels, Joyner signed a one-year deal with the Kansas City Royals in December 1991 for a then-remarkable $4.2 million. Even though Joyner ended up batting only .269 with nine homers that year, KC still picked up his options for the following three years, pushing aside prospect Jeff Conine in the process. Joyner stayed with the team until 1995, when he was traded with pitcher Aaron Dorlarque to the San Diego Padres for the versatile Bip Roberts and pitcher Bryan Wolff. Joyner helped San Diego reach postseason play in 1996 and 1998, but batted anemically both years, especially against the New York Yankees in the 1998 World Series, when he went 0-for-8.

Throughout his career, Joyner consistently hit around .300, but only hit over twenty home runs three times. With the face of the game changing around him, Joyner was hitting for average at a power position, and with the exception of three seasons, not producing runs at the rate a team would generally expect of out of its first-sacker. Additionally, Joyner dealt with nagging injuries during the 90's: pulled hamstrings, strained quadriceps, and sore shoulders all kept him from playing full seasons.

But despite the injuries and lack of power, Joyner repeatedly posted tidy batting averages and steady infield hands, a pillar of consistence. Additionally, the kind, practicing Mormon was popular with fans and teammates, and front offices often overlooked subpar statistics to keep him around.

The Padres traded Joyner along with Quilvio Veras and Reggie Sanders to the Atlanta Braves for Ryan Klesko and Jason Shiell during the winter meetings of December 1999. Though the Braves were actually after Veras and Sanders, the club had figured that Joyner would be an adequate backup for Andres Galarraga. Joyner reached a personal highlight that year, slamming his 200th home run on July 29, 2000, when Galarraga was on the disabled list. A couple of days after that, Joyner knocked his 2,000th hit, but was soon relegated to backup duty when Galarraga came off the bench.

In January 2001, clearly in his twilight years, Joyner signed with his original team as a free agent, hoping to replace the injured Mo Vaughn. After two and a half months, batting .243 with three homers, Joyner decided to call it quits. "I still felt great when I was on deck, felt great walking back to the dugout," the first baseman admitted. "It was what was in between that wasn't so great anymore."