| 27 Sept 19 | J. Meredith James Meredith |  MillwallMIL | → |  Perth GloryPER | Free |
| 27 Sept 19 | J. Meredith James Meredith | MillwallMIL | → | Perth GloryPER | Free |
| 08 Aug 19 | S. Morison Steve Morison | MillwallMIL | → |  Shrewsbury TownSHR | Undisclosed |
| 08 Aug 19 | L. Steele Luke Steele |  Nottingham ForestFOR | → | MillwallMIL | Loan |
| 31 Jul 19 | D. McNamara Dan McNamara | MillwallMIL | → |  Newport CountyNEW | Loan |
| 30 Jul 19 | J. McCarthy Jason McCarthy |  Wycombe WanderersWYC | → | MillwallMIL | Undisclosed |
| 30 Jul 19 | B. Bialkowski Bartosz Bialkowski |  Ipswich TownIPS | → | MillwallMIL | Loan |
| 30 Jul 19 | F. Onyedinma Fred Onyedinma | MillwallMIL | → | Wycombe WanderersWYC | Undisclosed |
| 23 Jul 19 | J. Molumby Jayson Molumby |  Brighton & Hove AlbionBHA | → | MillwallMIL | Loan |
| 23 Jul 19 | J. Molumby Jayson Molumby | Brighton & Hove AlbionBHA | → | MillwallMIL | Loan |
| 12 Jul 19 | J. Bödvarsson Jón Bödvarsson |  ReadingREA | → | MillwallMIL | Undisclosed |
| 09 Jul 19 | C. Mahoney Connor Mahoney |  AFC BournemouthBOU | → | MillwallMIL | Undisclosed |
| 09 Jul 19 | C. Mahoney Connor Mahoney | AFC BournemouthBOU | → | MillwallMIL | Undisclosed |
| 01 Jul 19 | D. Martin David Martin | MillwallMIL | → |  West Ham UnitedWHU | Free |
| 01 Jul 19 | M. Smith Matthew Smith |  Queens Park RangersQPR | → | MillwallMIL | Undisclosed |
| 01 Jul 19 | F. Fielding Frank Fielding |  Bristol CityBRC | → | MillwallMIL | Free |
| 01 Jul 19 | L. Gregory Lee Gregory | MillwallMIL | → |  Stoke CitySTK | Free |
| 01 Jul 19 | D. Martin David Martin | MillwallMIL | → | West Ham UnitedWHU | Free |
| 01 Jul 19 | R. Tunnicliffe Ryan Tunnicliffe | MillwallMIL | → |  Luton TownLUT | Free |
| 01 Jul 19 | S. Morison Steve Morison | MillwallMIL | → | Shrewsbury TownSHR | Loan |
| 17 Jan 19 | J. Brown James Brown | MillwallMIL | → |  Lincoln CityLCN | Loan |
| 16 Jan 19 | B. Marshall Ben Marshall |  Norwich CityNOR | → | MillwallMIL | Loan |
| 04 Jan 19 | A. Pearce Alex Pearce |  Derby CountyDER | → | MillwallMIL | Loan |
| 04 Jan 19 | B. Webster Byron Webster | MillwallMIL | → |  Scunthorpe UnitedSCU | Free |
| 04 Jan 19 | R. Leonard Ryan Leonard |  Sheffield UnitedSHU | → | MillwallMIL | Undisclosed |
| 01 Jan 19 | R. Leonard Ryan Leonard | Sheffield UnitedSHU | → | MillwallMIL | Undisclosed |
| 01 Jan 19 | T. Bradshaw Tom Bradshaw |  BarnsleyBAR | → | MillwallMIL | Undisclosed |