The SPFL League 1 club have received a nominal fee for a striker who made his first-team debut in March last year, at the age of 16.
Just over one year on and the teenager has been recruited by Sky Bet Championship club Huddersfield following a trial. He is expected to continue his fledgling career in the club's under-18s side, while his progress will be watched closely by academy manager Mark Willis, who is currently working as caretaker manager of the first team following Mark Robins' dismissal.
"We're delighted that a lot of hard work and perseverance has paid dividends in Liam's arrival at the club," said Lillis of a player who scored three goals for Airdrie last season. "It's testament to his talent that he has already played so much senior football at his age, but he will now take on a new challenge as he makes the step up to full-time.
"The academy technical staff liked what they saw when Liam was on trial with us; he has very clever movement off the ball and is a good finisher. As everyone knows, Liam has the ability to progress and make an impact on the first team."
It is not yet known who will be in charge of the senior squad on a permanent basis, although Steven Pressley has played down speculation linking him to the job. He is currently in charge of Coventry City.
"It's flattering to be connected with such a job but I am truly happy at this club," he said. "I've begun what I believe is a long journey developing the club."