Fast-bowler Mills makes the squad once more after an impressive T20 World Cup campaign earlier this year, which saw him take seven wickets in four matches before injury forced his withdrawal.
Garton is selected for the national team for the first time since his call-up to the One Day international series against Pakistan back in July, where he unfortunately had to isolate with the rest of the team without playing a match following an outbreak of Covid.
Paul Collingwood will stand in as head coach for the five-match series, which will be played in Barbados from January 22-30. Marcus Trescothick will join the squad as an assistant coach.
The squad includes 11 players who travelled to the United Arab Emirates for the T20 World Cup last November. Garton and David Payne are both uncapped players in the squad.
Collingwood said: "We have selected a strong squad with some serious batting power and a balanced attack as we begin preparations ahead of the T20 World Cup in Australia.
"The World Cup is less than a year away and there will be increased opportunities for the squad in the absence of those players who are with the Ashes squad.
"I have good memories winning a World Cup in Barbados and I’m really looking forward to going back there with this squad to face a very good West Indies who will test all aspects of our skills."
Squad: Eoin Morgan (Middlesex – captain), Moeen Ali (Worcestershire), Tom Banton (Somerset), Sam Billings (Kent), Liam Dawson (Hampshire), George Garton (Sussex), Chris Jordan (Surrey), Liam Livingstone (Lancashire), Saqib Mahmood (Lancashire), Tymal Mills (Sussex), David Payne (Gloucestershire), Adil Rashid (Yorkshire), Jason Roy (Surrey), Phil Salt (Lancashire), Reece Topley (Surrey), James Vince (Hampshire).
T20I Series, all matches to be played at the Kensington Oval, Barbados:
Saturday, January 22 – 1st T20I West Indies v England
Sunday, January 23 – 2nd T20I West Indies v England
Wednesday, January 26 – 3rd T20I West Indies v England
Saturday, January 29 – 4th T20I West Indies v England
Sunday, January 30 – 5th T20I West Indies v England