Connolly joined the Lions in January after his short spell with the Black Cats, but despite a bright start at the Den failed to earn a longer stay under boss Alex Neil.
And League One club Orient have now swooped to sign the free agent striker on a two-year deal.
Connolly said: “I'm buzzing to be here. I met with the gaffer last Monday, and straight away we both wanted to get it done.
"I'm delighted to be here, and the gaffer was a main factor in me coming here. I know the club just missed out on promotion last year, but we're hoping to go one better this year.
"I had my first session today, and I'm delighted that the transfer is done now. It happened quite quickly, which is always good for a player."
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Boss Richie Wellens said: “I tried to sign Aaron when I was at Swindon a few years ago, but he then scored a hat-trick against MK Dons in the EFL Trophy. Everyone was in for him after that.
“He’s talented, quick, good on the ball and skilful. He’s got an eye for goal, and he scored twice on his Premier League debut for Brighton. The ability is all there, and when I met him recently I got a really good vibe off him. I sense that there is a burning desire for him to do well.
“If we can get Aaron firing, I have absolutely no doubt in his abilities.”
Connolly burst through on to the first team scene at Brighton but endured struggles off the pitch, which he bravely opened up about when he joined Sunderland.
He made 22 appearances for Boro during a loan spell in 2022.
He joined the Black Cats in September last year, and spoke out in a revealing interview about his struggles with alcohol having spent a period of last summer in rehab.
When he left Sunderland, he told of how he'll always be grateful to the club and the fans for their support.
The former Republic of Ireland international will now be hoping to get his career back on track with the O's, who lost in last season's League One play-off final.
Martin Ling, the club's director of football, said: “Aaron is an excellent technical footballer, and we’re ecstatic that he has decided to join the project we are building here at Leyton Orient.
“He burst onto the scene when he was younger, and we know he has bags of natural ability. We’ve been on the hunt for a striker since the window opened, and Aaron fits the profile of what we were looking for to a tee.
“We’re really excited to have him at the club with us, and are looking forward to seeing what he can do in an Orient shirt.”