Dalby, who signed his first professional deal with the O’s in December 2016, made his debut for the club in a 3-1 win over Stevenage in the Checkatrade Trophy at the age of 16 years and 226 days, becoming the sixth youngest player to appear for the O’s.
The forward made his first league start on December 26th, 2016 at home to Crawley Town, scoring within the opening 4 minutes and the 18-year-old went on to make nineteen appearances for the O’s, registering several assists.
Director of football Martin Ling told the club`s official website: “I would like to wish Sam the best of luck in his future career and thank him for his efforts over his time at the Club. This was a good deal and opportunity for both the club and the player, and therefore was difficult to turn down.”
Ling`s opposite number at Leeds, Victor Orta, added “We are pleased to welcome Sam to Leeds United, we believe that he can be an important player for the future.
He has already tasted League Two and National League football, he is tall and strong, and we are confident that in a couple of years` time he can be competing for a place in our first team.
“In the immediate future Sam will join in with our under-23’s, who will help to develop him into the player we believe he can be.”
After sealing his move to, Dalby will be available for Friday's under-23 game against Barnsley at Oakwell.