Hugill has signed for Kilmarnock on a one-and-a-half year deal.
The move brings an end to the striker's six-year spell on the Old Trafford books.
The 22-year-old, who spent the second half of last season with Carlisle, has spoken of his emotions at ending his time with Man Utd.
"Thank you to everyone @manutd for the past 6 years," he wrote on Instagram.
"The players, the staff, the fans and everyone connected to the club who have made me love every minute along the way.
"I will always be beyond grateful for my time here and the memories will live with me forever."
Durham-born Hugill's deal at Killie runs until May 2027. He spent the first half of this season on loan with League Two club Barnet but recently returned to Man Utd.
The forward joined Carlisle in last January's transfer window under former head coach Mike Williamson.
He scored two goals in 17 appearances for the Blues.
Kilmarnock are second bottom in Scotland's top flight. They have other former Carlisle United connections in the shape of head physio Ross Goodwin and head of sports science Jake Simpson.
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