The 22-year-old never made a senior appearance for Brentford after moving there from Irish side Bray Wanderers, spending his time instead with the club's B team, where he made 30 appearances in 2024-25 despite injury problems and helped them win the Professional Development League title; he added a further 25 appearances the following season as the B team retained that trophy.
A former Republic of Ireland Under-21 international who scored twice on his debut against Hungary, McManus began his career in the Cabinteely FC academy before Bray Wanderers signed him and Brentford picked him up in December 2022. Primarily a left-back, he is capable of playing anywhere across the back four.
Speaking to Gills TV after the move, McManus said: "I am delighted to be here, it is somewhere I want to be and I'm looking forward to the season ahead.
"I am energetic, willing to work hard and am someone young and hungry. It's a really good group of lads and I'm ready to go. This is probably, physically, the best I have felt going into any season."
He will be available to make his Gillingham debut in their EFL Cup tie at home to Luton Town on Saturday. The move continues a busy summer of squad-building for manager Ainsworth, who had made no secret of wanting a new left-back after selling defender Sam Gale to Barnsley.
McManus becomes the tenth new arrival of the window as Gillingham look to end back-to-back bottom-half finishes in League Two. He arrives having previously trained regularly with Brentford's first team, including an appearance in front of 40,000 fans during a pre-season tour match against Benfica ahead of the 2024-25 season, and registered two assists last term while completing over 85% of his passes from left-back for the B team.