He was replaced by offensive coordinator Mark Helfrich, who will make his debut as head coach on Aug. 31.
Oregon was previously penalized by the NCAA in 2004 for a major violation involving the improper recruitment of a junior college player by an assistant coach. The university was put on probation for two years and the unidentified assistant coach was suspended without pay for a week and restricted from some recruiting activities.
The Ducks remained eligible for postseason play and did not lose any scholarships because of that violation, which occurred in 2003.
The latest recruiting violations are another hit to Oregon's image, but are unlikely to have an impact on the field for a program that's become an annual national-title contender
"It was a lengthy process," Mullens said. "We've been at this for two and a half years and as we said, there's been some negative recruiting around it. Obviously, the length of the process and the process itself had some punitive measures to it. In the end, I don't think the facts tarnish anything."