Lump sum inherited 401k distribution - K-1 or 1099R

That depends on what actually happened.  

Under one set of facts (probably more common), if you were the beneficiary of the 401k, you would become the owner of an inherited 401k account.  There would be certain rules about when and how much you needed to withdraw, and you would get a 1099-R from the plan administrator if you made withdrawals.

However, if no beneficiary was designated, or if the estate was the beneficiary, then the 401k plan might be cashed out by the estate and distributed as a lump sum.  In that case you get a K-1 from the estate, and that is taxable.  You don't get a 1099-R because you never had a 401k account (from this plan)--it was cashed out before it got to you.  That sounds like what happened.  

Thank you. This is just the answer I was looking for.  The K-1 was received and I reported the income on Schedule E.  

As Opus 17 suggests, the facts indicate that your mother's estate, not you, received the distribution from the 401(k) and you received distributable net income from the estate.