Train from Heathrow terminal 2 to St Pancras

I wouldn't take the train. The Picadilly Underground line goes to the St Pancras Kings Cross station in a little under an hour. If you take the train, it goes to Paddington, then you have to get the tube to St Pancras. The tube should get you there in plenty of time.

Can your question be rephrased thusly?

My flight is scheduled to arrive at LHR at 920 AM. I want to take a eurostar train that day. What is the earliest train I make ( i.e. pre-book tickets for) and have a reasonable chance of success?

If so, just getting to St Pancras, is not the whole issue. You need to be there in sufficient time to go through the boarding process and other formalities. Below is from the eurostar website

Standard and Standard Premier
On most routes
Please arrive at the station 45-60 minutes before departure.
This'll give you enough time to go through the ticket gates, security and passport control.
The ticket gates close 30 minutes before departure and if you're late, the train can't wait.

https://www.eurostar.com/us-en/travel-info/your-trip/check-in

Where are you flying from? I would calculate it as follows:

• If arriving on an intercontinental flight, allowance for plane possibly landing late: 1 hour.
• Disembarking, walking through endless corridors: 30 minutes
• Immigration (passport control): 1 hour, could take longer if you are really unlucky, but you already have a 1 hour contingency above.
• Luggage reclaim - 5 minutes. You have already spent so much time in immigration your bags are waiting for you.
• Customs - zero, random stop only.
• Walking to tube station, buying ticket - 10 minutes
• Tube (Piccadilly Line) to St Pancras - 1 hour.
• Walking from tube station to Eurostar - 5 minutes
• Time you need to be through ticket barriers at St Pancras before departure time - 45 mins to 1 hour

Total: 4 hours 50 Minutes
I would look for a Eurostar departue 5 hours after your scheduled landing time.
Eurostar timetable here: https://content-static.eurostar.com/documents/460888_Timetables%20Core%20Destinations_Issue%2083_EN_0.pdf

Well, you have been given lots of confusing info.

To get from Heathrow to St. Pancras, take the Piccadilly line (known as the Tube or Underground, not the train) to Kings Cross. It is across the street from St. Pancras. It will take about an hour and 15 minutes at that hour maybe less. No changes. And since you are getting on at the beginning of the line you should be able to get a seat.

If you are a U.S., Canadian or Australian citizen you can now use the egates at Heathrow immigration. That should speed things up.

Allowing 4-5 hours makes sense for any delays that might occur. I would lean closer to 5 hours. And if you are early you can always grab coffee or breakfast at St. Pancras.

Thanks for all the replies. I sincerely appreciate the help.