As a mental health therapist (1099) can I claim mileage if I have an office in a building and then I also do administrative work at home with an office set up there?

No you cannot ... since you have a office where the majority of the work happens you cannot also have an office in the home so you cannot claim the commuting mileage anymore than an employee can. Sorry.

Thank you for that. But what if I do virtual sessions at home as well?

Still no ... you pay rent somewhere else so an OIH is not allowed. It is not your Principal place of business ... read the rules here :  <a rel="nofollow" target="_blank" href="https://www.irs.gov/businesses/small-businesses-self-employed/home-office-deduction">https://www.irs.gov/businesses/small-businesses-self-employed/home-office-deduction</a>

So I don't pay rent elsewhere as I am a 1099 at a place of business. Does that change anything?

Not really ... but how do you get away with not paying rent ?  Do you own the building ?  If so that is the same as renting ... you still have costs to keep up the office at your principal place of work.

I work for a company that pays the rent.

Wow ... so you are an employee ?

Or should be an employee but they treat you as a sub contractor.  

read the rules in the link from the IRS ... you will have to decide if you have a valid office in home that will allow the mileage deduction which is the most audited section of the tax laws.