McLaughlin finished third in St. Petersburg while team-mate Josef Newgarden took the win, but both drivers were disqualified for illegally being able to use push-to-pass on the restarts.
Will Power, who drives the third entry for Team Penske, received a 10-point penalty but was not disqualified after it was discovered he did not use the overtake system on the restarts.
Defending his actions and those of his team, McLaughlin said: “First and foremost, I am proud to be a member of Team Penske.
“I fully stand with everyone one of my team-mates. Simply put, a mistake was made.
“I have the highest level of integrity and it is important to protect both my own reputation and that of the team.
“I was not aware of the situation with the software. In this instance, I used a single, very brief (1.9 second) deployment of push to pass in a section of the track exit of Turn 9 where it is typically utilised throughout the race.
“I hit the button out of habit, but I did not pass any cars nor did I gain any time advantage.
“The data, which IndyCar has, confirms all of this information. While I accept the penalty, I want to be clear that I did not gain an advantage over my competitors.
“IndyCar’s competition is the best in the world and I would take no pleasure in achieving success in any way other than honestly.
“We will all press forward from here and focus on the task at hand this weekend in Barber.”
The updated results, which came 45 days following the checkered flag at the 1.8-mile, 14-turn temporary street circuit, saw Arrow McLaren’s Pato O’Ward promoted as the race winner, with Power elevated to second and Andretti Global’s Colton Herta third.
Two-time IndyCar champion Power posted a statement of his own on Wednesday on social media.
"I was disappointed to learn about the penalty that we received this week from Indycar," he wrote.
"There was an oversight by our team and I was unaware of the situation until it was brought to our attention following Long Beach.
"As per the rules, I did not utilize the P2P capabilities during any start or restart during the St. Petersburg race.
"While I accept the penalty, I want it to be known that I did nothing wrong and followed the rules.
"Now we move forward and my focus is on Barber."
The outcome is a significant hit to both Newgarden and McLaughlin in the championship standings. Newgarden, who was the overall leader after two points-paying rounds, has now dropped to 11th with 34 points.
McLaughlin has fallen to 29th in the standings with only five points after enduring a mechanical issue that led to finishing 26th last weekend in Long Beach.
In a separate statement, Team Penske president Tim Cindric added: “Team Penske accepts the penalties applied by IndyCar”.
