A news release stated Mark, an All-America punt returner and second-team All-Big Ten running back in 2012, is transferring because of personal reasons to play closer to his home in Houston. Mark learned of his university-issued suspension in June and appealed, but he told reporters Tuesday, "There's no point in pouting. I'm going to embrace it."
New developments that surfaced earlier this week prompted Mark's decision to transfer, a source told ESPN.com. He missed most of the 2013 season with leg injuries but received an extra year of eligibility from the NCAA.
Mark rushed for 1,366 yards and 12 touchdowns and had two punt return touchdowns during a breakout year in 2012.
"Northwestern has been an indescribable experience for me," Mark said in a statement. "It has been my home for four years, and has molded me into the man I am. I'm one class shy of the Northwestern degree I've worked so hard for, and I will graduate. I'm devastated to leave my second home, but life is full of challenges and I've been presented with another one. Right now this is what is best for me and my family."
On Thursday coach Pat Fitzgerald said Mark could have stayed on the team, but he decided to transfer after the two spoke on Tuesday.
"You're saddened by it," Fitzgerald said. "It's one of those situations where you lift him up. We're always going to be here for him. At the same time, it's what's right for him and his family, and we'll move forward from there.
"We were in discussions [of him staying]. He made the decision that was right for him and his family. We fully supported that."
Fitzgerald added: "One of the last things I said to him the other night, 'We're always going to be here for you. The challenge, though, is really on you. You've got to speak up, you've got to reach out and you've got to help us help you.' Hopefully he will."
Northwestern also announced wide receiver Christian Jones will miss the 2014 season because of a knee injury suffered during practice. Jones led Northwestern with 668 receiving yards and finished second with 54 catches last season.
The senior has a redshirt available and is expected to return in 2015.