If you have to work, that's fine.
Otherwise, it's only servile (manual) work done without necessity that is a mortal sin, and then only if it's 'between two and three hours' or more (cf. Henry Davis SJ, Moral and Pastoral Theology, 1930.)
Yes, many Catholics are unfortunately forced to work on Sunday. And of course we should consider it "forced by necessity" and never by our own choice.
If you can work Saturdays, Friday nights, or any other time instead, you should be willing to (and let your boss know that). Don't ever embrace your necessary Sunday work as an excuse -- ask yourself if you truly like it, or if it truly bothers you.
Be honest with yourself, and be honest with God.You should make a priority out of getting every Sunday off for Mass. Even if it means making less money, or if it meant changing careers.
A Catholic should put his Faith first. The Mass has infinite graces and we should be anxious to partake of these graces.
If next Sunday every Tridentine Mass was giving each parishioner a steak dinner , plus somehow infusing knowledge into their brains (how to play various instruments, how to work with tools, various foreign languages), or if your favorite hero was going to be at your local Tridentine Mass chapel next Sunday, or if ALL your friends and family were having an incredible party in the parish hall after Mass next Sunday, would you be just as content with your situation that "it's necessity which keeps me away?" or would you be using every means, fair or foul, to get Sundays off?
Those are the questions we must ask ourselves.
But yes, it's not sinful if your employer requires you to work on Sunday.
Just try to find a better employer!
P.S. For those forced to work on Sunday, you probably get 2 other days off during the week. One of those days should be your "Sunday" where you abstain from servile work, take care of your soul (Mass if available), etc.
P.P.S. Catholic should also become "model employees" so they become indispensible workers, and thus the boss thinks twice about losing them by not giving them Sunday off. Imagine some hard working, model employee who tells his boss, "I love working here, but I must keep looking for another job in my spare time because I MUST have Sundays off. No other perk tempts me. My only career goal is getting Sundays off." If that boss could swing it, you know he'd give you at least SOME Sundays off.