Brophy joins Cambridge from Leyton Orient on a three-year contract, and is their second signing of the close season following Lloyd Jones’ arrival earlier this week.
He told his new club’s official website: “It was an important decision.
“There were two key factors – I wanted to come back into League One and I wanted to find a place and a gaffer that I can relate to and that I felt he had trust me in and I had trust in him.
“I am really excited now and can’t wait to get started.”
Cambridge boss Mark Bonner said Brophy was “a character we have warmed to instantly”.
Brophy spent four seasons with Orient. He joined them from Swindon having begun his career in non-league with clubs like Broadfields and Edgware Town.