Looking at a United flight from Washington Dulles to Lisbon with a connection in Munich. Do I go through Immigration in Munich or Lisbon? Asking because if it is Munich I will need a longer layover. I've searched here and the web but all I see are conflicting answers. Thanks!

You go through Immigration at the first stop -- Munich, and Customs at the final destination -- Lisbon.

If you are all on one ticket with any bags checked through or carry-on only then the layover time allows for you to go through passport control in Munich and make your onward flight.
Customs in Portugal is a non-event unless you have something to declare.

Munich has a minimum connection time of 40 minutes, and I've seen itineraries with connections in that range. Be careful with those. That airport used to be very efficient, but we've been through there a few times in the last year or so when there were only a couple of passport control agents on duty and the lines were very long. Depending on your arrival and departure gates, you may also need to take a short train between the main and satellite gates in Terminal 2. You shouldn't have to go through security on that connection.

Like ms_go, my experiences lead me to be leery of the minimum connection times listed for many airports. I generally assume I'll need at least 2 hours, no matter what the routing itineraries suggest.