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.