5:33PM BST 07 Jul 2011
Brown made over 300 appearances for his home town club since signing his first professional contract in 1996, but only 15 of those came last season.
The 31-year-old, capped 23 times by England, has had his fair share of injury problems over the years, although it was a lack of first team football when fit that persuaded him to leave.
The versatile defender has signed a four-year contract at the Stadium of Light and will be joined by another Old Trafford Academy graduate, John O'Shea who also signed for the Black Cats today.
"It has been OK getting my head around leaving United,' Brown said. 'I am older now so you deal with these things.
'It was disappointing not playing as much as I could have at Manchester United in the last couple of seasons. Just to know that I will be back in playing and focusing on that makes it better.