The California League All-Star homered off Stockton starter Fabian Williamson in the first inning of Sunday's 4-2 loss to the Ports.
The solo blast cleared the visitors' bullpen in left field at Banner Island Ballpark.
"I knew that I had 99 [RBIs], but I didn't know that I was the first," he said. "It feels good."
The 22-year-old corner infielder reached the milestone ahead of Dan Johnson, who drove in 95 runs for the Triple-A Durham Bulls before he was promoted to the Major Leagues last week. Yankees prospect Brandon Laird has combined for 95 RBIs with Double-A Trenton and Triple-A Scranton/Wilkes-Barre.
Poythress, the Mariners' second-round pick in the 2009 Draft, said that while the individual accomplishment feels good, the success of the team is the most important thing in his mind when he steps into the batters' box.
"Driving in a lot of runs in the middle of the order is definitely a goal you set for yourself at the start of the season," he said. "That is the nice thing about RBIs. The more guys you drive in, the more it helps the team. They go hand in hand and you just want to do anything you can to help the team win.
"It is reflective of the guys in front of me. [Kyle] Seager is batting .350 leading off and [Johermyn] Chavez, [Shaver] Hansen and [Eddy] Martinez-Esteve are swinging the bat unbelievably well. I feel good at the plate, and having guys on in front of me helps. You can never say enough about how much that makes it easier."
Seager made headlines when he compiled a 32-game hitting streak from June 19-July 27.
Poythress, has been on a tear since the California-Carolina League All-Star Game on June 22, hitting .315 with 34 RBIs in 37 games. In fact, the University of Georgia product has not gone more than one game without driving in a run since July 15.
Among the highlights during that stretch was a seven-RBI effort on July 17, the second time he drove in seven runs against Inland Empire in less than six weeks.
"I am just trying to do the same thing every day and have a good at-bat every time I step into the batter's box," Poythress said. "I try to take advantage of the opportunities when they are there, like hitting a sac fly when there are guys on third."
That consistency has has been on display all year: he had at least 20 RBIs each of the first four months.
Sunday's setback dropped the Mavericks into a three-way tie for first place in the California League's South Division. High Desert missed the playoffs in the first half of the season.
"We are having a good season and we are swinging the bat offensively and we are a team that can make a run here [at the playoffs] by the end of it," Poythress said. "That is our goal now."