In 1990, the Griffeys became the first father-son pair to play in the same Major League lineup

Under most circumstances, a father and son working for the same organization would be the result of nepotism. That nepotism doesn't always carry a stigma, such as when a son takes over the family's local pizza joint. In the instance, however, of a son being hired to work at the same law firm as his father, the possibility of nepotism or favorable consideration carries the weight of an accusation.
In baseball, unlike these other cases, a father and son playing for the same organization is nothing other than pure fun.
After thousands of games, many generations of players and dozens of father-son duos have passed through the well-over 100-year history of Major League Baseball. But, on Aug. 31, 1990, Ken Griffey Sr. and Jr. did something no father and son had ever done before: play together in the same lineup.
Aug. 31, of course, is five months into the season and a month after the Trade Deadline. So, why did it take so long that year for the Griffeys to wear the same uniform in a game?
At age 40 Griffey Sr. was in the third year of his second stint with the Reds, but, on a team with World Series aspirations, his .206/.235/.286 batting line wasn't cutting it. So, on Aug. 18, the organization gave him a choice: retire and go out on his own terms or the Reds would find some other way to part ways. Although he initially chose to retire so that he could play with his 20-year-old All-Star son on the Mariners, he soon learned that retirement came with a 60-day waiting period before signing with a new team. He therefore decided to unretire and accept his release from the Reds.
After clearing waivers, the elder Griffey signed with Junior's Mariners on Aug. 29. On his arrival, Senior expressed concern that his presence might disrupt his son's brilliant sophomore campaign. From the Seattle Times:
"Junior and I have always been so competitive," the father said. "He's always wanted to outdo me. I hope he doesn't try to press so hard. I hope it's a positive for everyone."
He made his debut two days later at the Kingdome when the Mariners hosted the Royals. Senior got the start in left field while his son was just to his left in center field.

Not only were the Griffeys next to each other in the outfield, but in the lineup as well. Griffey Sr. found himself batting second in front of Griffey Jr. batting third in the order.

Neither Griffey saw any first-inning action in the field, as the Royals were unable to hit a ball beyond the infield off Mariners starter Randy Johnson. But, they were due to bat second and third in the home half of the inning.
After Harold Reynolds led off with a flyout to center field. The elder Griffey came to the plate and hit a single up the middle. The younger Griffey followed and lined a single to right field.

They both came around to score in what would be a three-run first inning for the Mariners on their way to a 5-2 win.
However, aside from a second-inning walk by Griffey, the father, those back-to-back first-inning singles would be the extent of the Griffey offensive output in the duo's debut.
The father-son pair spent over a month together in Seattle and made their share of highlights together in that time. Most notably, on Sept. 14 against the Angels, they did better than their back-to-back singles in the debut: They hit back-to-back dingers.

Less notably, but not necessarily less amusing, Junior disregarded his father's claim to a fly ball and recorded the out himself, thank you very much:

When Griffey Jr. was inducted into the Hall of Fame in 2016, he reflected on his memories of playing with his father in 1990:

Only Tim Raines Sr. and Jr. have since joined the Griffeys in accomplishing this feat, but, of course, the Griffey pair will forever remain the first to suit up for the same team. With back-to-back singles in their first at-bats together and back-to-back dingers later in the season, odds are that, even if other father-son duos join them, the Griffeys will remain the best to ever do it.