Greenwood, 21, was arrested in January 2022 but charges against the England international were dropped in February this year.
United had then launched an internal investigation in which they would assess Greenwood's future while the forward had been training individually.
The probe found Greenwood did not commit the offences he was charged with but came to the conclusion it would be best for all parties that his future was away from Old Trafford.
"Manchester United has concluded its internal investigation into the allegations made against Mason Greenwood," a club statement read.
"Our process commenced in February 2023, following all charges against Mason being dropped. Throughout, we have taken into account the wishes, rights and perspective of the alleged victim along with the club's standards and values, and sought to collate as much information and context as possible. This has required us to proceed with sensitivity and care to obtain evidence not in the public domain, including from those with direct knowledge of the case.
"Based on the evidence available to us, we have concluded that the material posted online did not provide a full picture and that Mason did not commit the offences in respect of which he was originally charged. That said, as Mason publicly acknowledges today, he has made mistakes which he is taking responsibility for.
"All those involved, including Mason, recognise the difficulties with him recommencing his career at Manchester United. It has therefore been mutually agreed that it would be most appropriate for him to do so away from Old Trafford, and we will now work with Mason to achieve that outcome."
In a statement of his own, Greenwood accepts he "made mistakes" and took partial responsibility, but explained he did not do the things he was accused of.
"I want to start by saying I understand that people will judge me because of what they have seen and heard on social media, and I know people will think the worst," he said.
"I was brought up to know that violence or abuse in any relationship is wrong, I did not do the things I was accused of, and in February I was cleared of all charges. However, I fully accept I made mistakes in my relationship, and I take my share of responsibility for the situations which led to the social media post.
"I am learning to understand my responsibilities to set a good example as a professional footballer, and I'm focused on the big responsibility of being a father, as well as a good partner.
"Today's decision has been part of a collaborative process between Manchester United, my family and me. The best decision for us all, is for me to continue my football career away from Old Trafford, where my presence will not be a distraction for the club. I thank the club for their support since I joined aged seven. There will always be a part of me which is United.
"I am enormously grateful to my family and all my loved ones for their support, and it is now for me to repay the trust those around me have shown. I intend to be a better footballer, but most importantly a good father, a better person, and to use my talents in a positive way on and off the pitch."
United chief executive Richard Arnold said Greenwood's reintegration back into the squad had been "considered and planned for" but the progression of the process "evolved" his view and decision.
Greenwood, who last played on January 22nd, 2022, emerged through the club's academy and made 129 appearances scoring 35 goals.
He has one cap for England to his name, which came in September 2020 in the Nations League.
