After 90 minutes of evidence and another half hour of jury deliberation on Tuesday night, tribunal chair Jeff Gleeson KC gave a lengthy verdict that upheld the charge.
Mansell shook his head when Gleeson announced the ruling and the Tigers will decide by late Wednesday morning whether to appeal.
This was widely seen as a test case after the AFL issued a memo last month warning players against pushing opponents into marking contests.
The Tigers argued Mansell was only trying to push off St Kilda opponent Liam O'Connell during Saturday's game.
O'Connell instead was pushed into the path of Tigers key forward Tom Lynch and Saints defender Anthony Caminiti.
The impact concussed O'Connell and he is set to miss one game.
Match review officer Michael Christian graded Mansell's incident as careless conduct, severe impact and high contact.
The Tribunal rejected Mansell's evidence he did not see Lynch and Caminiti running in to mark the ball, saying the vision was "not consistent" with this.
Gleeson also said Mansell also should have known other players would try to contest the mark.
"The force of the push is a significant factor here. It went well beyond what a reasonable player would consider prudent in the circumstances, particularly the circumstances that the push was in the direction of the path of the ball," Gleeson said.
"Mansell breached his duty of care by pushing O'Connell with such force, with the path of the ball directly in the direction of oncoming players.
"Mansell's eyes were not on the ball. He was not attempting to mark the ball and he did not push O'Connell to immediately gather the ball.
"His evidence that he hoped O'Connell might drop the mark is not a satisfactory explanation or justification for his conduct."
The Tribunal also rejected Richmond's argument West Coast player Reuben Ginbey had not been charged for a pre-season incident where he pushed Tigers youngster Sam Lalor into a marking contest, leaving him with a fractured jaw and concussion.
The Ginbey-Lalor incident was among a string of pre-season incidents that prompted last month's AFL memo to clubs warning players about pushing opponents into marking contests.
"We do not find that the Ginbey example is sufficiently comparable to change our view that this incident constituted rough conduct," Gleeson said.
Earlier on Tuesday, AFL chief executive Andrew Dillon said Mansell's case differed from other incidents that had escaped match review sanction.
"The one that's in front of the Tribunal today has got a number of key differentiators to the other ones ... and that will play out tonight," Dillon told reporters in Adelaide.
Dillon refused to label Mansell's tribunal as a test case, saying he was reluctant to comment on individual hearings.
"We don't want to necessarily talk here about that individual incident as it's before the Tribunal and through the MRO," he said.
"What I will say is that the whole MRO and Tribunal process is about protecting the health and safety of our players.
"That has always been the forefront of what we're trying to do.
"So whether it's a test case or not, I think that it now gives the opportunity for Rhyan to put his case forward and we await the outcome."
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