Coombes signed for Billericay Town on Friday for an undisclosed fee after a great season for the Wings in the National League South.
Steve King acted quickly to resolve the issue with Ebbsfleet forward Mills arriving on a year’ loan.
He also added two other new players, powerful midfielders David Ijaha and Arnaud Mendy.
Mills wants to be in the goals next season to make sure Coombes’ departure isn’t felt.
“Obviously I know the gaffer very well from our time at Whitehawk and I feel that I played my best football under him during our time there, scoring nearly 60 goals during two seasons,” he told Welling’s official website.
“Looking at the squad, I know a lot of the boys like JP Kissock, Dave Ijaha and Ach (Anthony Acheampong) from playing with or against them over the years and definitely think that the squad is capable of promotion under the most successful manager at this level.
“Personally, I’m excited to get playing again after a stop-start season last year and I’ll be looking to play 40-50 games like I’ve done in previous seasons and set 20 goals as a benchmark.”