Is working under the table legal?

I've mowed lawns, fixed computers and given massages to people. I only accepted cash. Is that legal if I didn't report this in for my taxes?
Update: How would anyone know? And can I use these occupations to put on my resume for new jobs?
Update 2: How easy is it to find someones work history?
Update 3: How much do you have to make before they start caring?
Update 4: 400 a month or year? I'm assuming year?
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