I earned less than $50k annually, on and off, for many years, while we were building our stash. So did spouse, though the timing varied because we both did a lot of freelance work. Together, we probably always came in at over $50k, though.
We've had some strokes of luck, like buying a house at an opportune time. We also never fell into the credit-card trap, so we didn't start out with big piles of debt to pay off. However little we earned, we always tried to save some. It wasn't always possible, but we never really "grew" our lifestyle as our income increased.
We did most of our big earning early on and had a financial planner invest the surplus, and then let that money grow while working enough not to touch it, and occasionally to add to it. We no longer use a financial planner; I did the research and took over that role myself.
Having a big standalone freezer has also been a big help. That freezer is just about full, partially with home-cooked meals frozen into serving sizes for work lunches. We rarely eat out, partly because of food sensitivities, and partly because of cost. And we're both kind of homebodies, so not a lot of entertainment expenses for concerts, movies, parties, etc.
Also working in IT helps because neither of us needs an expensive business wardrobe. We do spend on computers, video games, and comic books, but that's another story.