Is a 40 minute transfer in Munich enough time? (Schengen to non Schengen)

Both flights are on Star Alliance airlines which means they will use Terminal 2. While your United flight will definitely use a jetbridge, it is possible for the Venice flight to be at a remote gate, using a bus to transfer you from the aeroplane to the terminal. The bus should drop you off at a main terminal (i.e. not satellite terminal) gate.

Your plan of action is to move to the middle of the terminal building where you can transfer from the Schengen (G) gates to the non-Schengen (H and L in the satellite) gates. The official airport map gives a walking time of 7 minutes from the furthest gates to the security/immigration checkpoint in the centre. In the centre, go upstairs following signs for connecting flights and passport control. Once you pass through there, you will need to navigate to your gate.

As your destination is the United States, it's possible for your departure gate to be in the US departures special area on one end of the (smaller) satellite terminal. However, checking today's departure it was actually at gate H08, i.e. in the main terminal building. Again, getting to the most distant part of the main terminal takes 7 minutes according to Munich airport's official map. Getting to the satellite (the entry to the underground people mover is right ahead of exit immigration) and to the most remote gates there should also not take much longer than that, although they do not give time estimates.

With this, you still have 20 minutes to stand in line awaiting your turn at exit immigration assuming the worst. If you hold EU passports, the time required should be much shorter. So yes, the connection is absolutely doable, when on one ticket.

Furthermore, assuming your inbound flight is delayed (and that this is, indeed, a single ticket connection), do speak to the cabin crew before landing and they will do their best to help you make it. Short connection times between Star Alliance flights is what Munich airport considers one of its selling points. Remember that Lufthansa (the airline that operates terminal 2) is also Star Alliance so in principle all Lufthansa staff present should also be available to help you if you are suddenly running short of time unexpectedly.

In addition, if this is one ticket and the Venice flight is so badly delayed that you cannot make it, it is the airline's responsibility to get you to your destination. Naturally, this is a hassle and may result in an additional transfer or a hotel stay and 24-hour delay, but it is good to keep that in the back of your head.

If you are going to attempt this on two tickets it is still possible but I would advise against it as (a) you would be on your own if things go wrong; (b) in case you have checked luggage you probably would need to retrieve and re-check it, causing your buffer to disappear like ice in the sunshine; and (c) it's readily available on a single ticket so you're buying a load of hassle for little benefit.