But the 24-year-old, whose spinal cord and vertebrae are closer together than is normal, set his sights on becoming the best Twenty20 player possible.
The Sussex fast bowler, who regularly bowls in excess of 90 miles per hour, is in the squad for England's T20 game against Pakistan on Wednesday after making his debut against Sri Lanka in July.
Mills' condition means that he is unable to play in longer formats of the game, as lots of bowling can aggravate his spinal cord. He has not bowled with a red ball for 18 months and it is unlikely that he will return to multi-day cricket.
"If this is going to be all I am doing now for the rest of my career, I want to do it right. I want to play in the big games and play to the highest level that I can," Mills told BBC Sport.
"I miss having a short leg, three slips and a gully sometimes, but on the flip side of that I can't say I miss doing 96 overs in a day.
"I am at peace with it, it's been 18 months since the reoccurrence of my back injury. I'm over it and I am grateful I can still play and I can still play to a good level."
The Yorkshire-born paceman played in 12 of Sussex's 14 T20 Blast group matches this season, taking 15 wickets at an average of 19.73.
"You want to be renowned as one of the top players. As soon as a tournament comes up or a team is being selected, you want to be one of the guys immediately spoken about," he added.
"I'm still a few years off that, I've only just turned 24. I'm not naive thinking I'm there yet but if I am only going to do this for the next few years then that's what I want to work towards being.
"I am quite lucky that when I got injured again I was a better Twenty20 bowler than I was a four-day bowler," Mills said. "If it was the other way round, then I probably would've had to think a lot harder about retiring.
"It's given me a chance to accelerate my development. Everything I do now in terms of cricket is to be the best Twenty20 cricketer I can be.
"It is hard for the other guys who have to flip between playing County Championship, 50-over and Twenty20 cricket - they've got to practice three different skills whereas I just practice one thing."
Although the condition does not affect the bowler on a day-to-day basis, he has to manage it to prolong his career.
Therefore, Mills has had to hone his training and approach in the build-up to games to ensure that he does not miss time.
"I haven't changed anything with my bowling, just away from it, making sure all the other stuff I do gives me the best chance of doing much as possible and to the best of my ability," he added.
"It was a lot of trial and error, I did a lot of work on running, with my back problem I need to put as little stress as possible on my back, so there is a lot of technique work. I don't lift any heavy weights any more."

The 21-year-old, part of Britain's C1 team event winners on Friday, finished more than two seconds ahead of the Czech Republic's Tereza Fiserova in Tacen, Slovenia on Saturday.
"To win two golds this weekend is really exciting," said Woods.
It was a second European singles gold medal for Woods after her 2015 success.
You could have forgiven Tymal Mills for retiring and moving on from cricket when he was diagnosed with a congenital back condition in 2015.
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Britain's Kimberley Woods won a second Canoe Slalom European Championships gold medal in two days with victory in the C1 women's canoe singles.