I'm 86 and still working full time and contributing to my employer 401k. Do I have to take an RMD?

[Carl]

Once you reach age 70½, you are required by the IRS to take annual Required Minimum Distributions (RMDs) from your retirement accounts. You need to calculate your RMD for each IRA separately, but you have the flexibility take your total RMD amount from either a single IRA or a combination of IRAs. However, RMDs from Qualified Retirement Plans or Inherited IRAs must also be calculated separately, and can only be taken from their respective accounts. You do not need to take RMDs for Roth IRAs.

[macuser_22]

The OP was asking about a 401(k) not an IRA - the rules are different.

For a defined benefit plan such as a 401(k) you must take the first RMD by April 1 of the year following the later of the year you turn 70 ½ or the year you retire (if allowed by your plan). If you are a 5% owner, you must start RMDs by April 1 of the year following the year you turn 70½.

You need to check with the plan administrator.  If you are still working and 86 then I would assume that the plan administrator would have already informed you if RMD's were required when you turned 70 1/2.