I see nothing wrong or odd with the following sentence and its meaning.
I didn't use to smoke
This statement means I was a non-smoker in the past. It implies that today I smoke on a regular basis. When we want to contrast a repeated action in the past that is no longer true in the present, used to, is a perfectly valid construction. Compare the same sentence in the affirmative
I used to smoke
This means I was a smoker in the past, but now I do not smoke. To make the past simple negative form in English use did + infinitive verb
I didn't smoked I didn't smoke
He didn't played football. He didn't play football
We didn't went to the match We didn't go to the match
Likewise with used to, the negative form in the past simple is didn't + use to
The difference between didn't use to and didn't used to is not noticeable in speech but this error (and it is an error, I don't care how many instances Google says otherwise) is replicated in writing.
The story is a little different with the second example (corrected)
I didn't use to work for McDonald's.
Here the speaker is saying he didn't work for an extended period for or in a McDonald's restaurant. However, this implies he is working for that company today.
A: Bla, bla, bla... What are you doing now?
B: I work in McDonald's
A: Really? But didn't you leave college with a Bachelor's degree in X?
B: Yeah... Listen I know it sounds weird, I didn't use to work in McDonald's but now I do. It's a long story.
Macmillian Dictionary says:
Used to only exists as a past tense. Questions and negatives are
usually formed with 'did' + use to (with no 'd'): Did you use to work
here? ♦ We didn't use to earn much. The spelling 'did used to' is
sometimes used, but many people think that this is wrong. In formal
English, negatives are often formed with used not to:
They used not to allow shops to be open on Sundays. The short forms usen't to and
usedn't to are sometimes used, but they sound rather formal and
used for saying what was true or what happened regularly in the past,
especially when you want to emphasize that this is not true or does
not happen now
- I used to enjoy gardening, but I don't have time for it now.
- They always used to ring me and say what they were doing.
- Where did you use to live before you moved here?
- I didn't use to like him, but now we're good friends.
- Customers didn't use to want to shop from home.
If you want to substitute "didn't use to" say never.
I never smoked but now I do
I never worked for McDonald's but now I do
Alternatively, as suggested by @Peter Shor in the comments below
I never used to smoke
I never used to work for McDonald's