The Hatters signed Millenic for an undisclosed fee from Exeter at the end of the window transfer window.
The 24-year-old arrives at Kenilworth Road after scoring 12 goals in all competitions during the 2024/25 season for the Grecians, nine of which have come in League One, including seven in his last eight matches.
Alli adapted well to life in the Football League following his move from National League side FC Halifax Town last January.
A player who operates primarily as a left-sided forward but can also play centrally, he scored seven goals in 21 matches last term for the Shaymen before his move to Devon, where he then found the net four times.
After starting out in the youth system at Bury, the Irishman established himself in non-League football with spells at Padiham, South Shields, Workington, Ashton United, Stockport County, Chorley and Halifax, where he also won the FA Trophy in 2023.
Alli signed for the Hatters on 30th January 2025 and made his debut alongside other recent signings Thelo Aasgaard and Kal Naismith as a substitute against Sheffield Wednesday in a 1-1 draw that brought a five-month away losing streak to a close.
Then on 12th February, he made his full debut in a 2-0 defeat at Sunderland before going on to total 16 appearances before the end of the season.
Alli scored four goals in that time, one of those being against Stoke where he netted a late equaliser to salvage a valuable point as the Hatters battled against relegation, before firing in the winner at Derby on Good Friday.