Travel from Heathrow to Eurostar station at St Pancras Station

If you need to get to St Pancras, it really makes no sense to go to Paddington. The Picadilly Underground line goes from Heathrow to Kings Cross in about an hour. Then it's a few minute's walk to St Pancras. You would want to allow at least an hour to deplane, clear Immigration, and get to the Underground station. And you need to be at the Eurostar queue for check in and Passport Control (exiting UK + entering France) 90 minutes before departure. I'd add a minimum fudge factor of an hour in case of delays. So that's a minimum of 4 1/2 hours from arrival.

Thank you for the help. I have never been to London so I was really at a loss for the best way to manage this transfer.

So you are flying into London having never been there before and them immediately departing without even looking around?
Please tell me you will be back in London at the end of the trip?
BTW why not fly into wherever you are going such as Paris? Save yourself a lot of time and effort,

Does it really matter? I've only ever landed at Dublin or London (oh yeah and Reykjavík), every single other European city I've visited I've either arrived by train or ferry and London has been my gateway city to Europe. Personally I enjoy the slower plodding travel and the time and effort is part of the enjoyment. But it does not matter the OP's plans, they only asked about getting from LHR to St Pancras.

OP there's the Elizabeth Line, Heathrow Express, or the Piccadilly line. Piccadilly line is the most straight forward direct line to St Pancras Station. At about 8am It'll be about a 60-65 minute journey from Heathrow to St Pancras and I think about £5.60. The other two options will cost more, involve transfers, and save 10-17 minutes. But select the option that best suites you. My only advice is to account for time clearing immigration, leaving the airport, journey time into London. THEN ensure you account for and add the necessary time to clear border formalities for Eurostar.

Looking at the OP's other post they are going for all but 2 weeks stay in France, and originally intended to fly to and from Paris. So presumably is using Eurostar for the experience, as a matter of choice.

As the OP is travelling in the morning peak, when the tube will become pretty busy on it's way into London, as will the Elizabeth Line there is a lot to be said for using Heathrow Express to Paddington then a taxi to St Pancras- both on speed and comfort grounds. There are currently £15 Heathrow Express tickets on 12 September so the cost is barely more than the Elizabeth Line.

There is maybe a £15 cost penalty on that route over the tube- which really is not significant in the overall cost of the trip.

With Eurostar check in times, even using HEX I would not be happy with a Eurostar departure time before 11am, preferably closer to 12 noon.

As of this morning the 1131 Eurostar is £74 and the 1231 is £63.

Taking everything together I would opt for the 1231 train. If I have time to kill at St Pancras then there are far worse places to do that.

No clue where OP is coming from, but early Sept is still prime hurricane season, so if the originating flight is from a Gulf or East coast airport, weather could be a consideration and cause a significantly delayed arrival in London.

I do not understand why OP is choosing to make his trip more complicated and fraught with peril than is necessary.

Others have already outlined the rail options; my only advice would be a Eurostar reservation no earlier than 2PM. If all goes well. you have time for an unhurried lunch; otherwise you have some cushion for 'oops'.

Without baggage to collect if I were in your position I'd look at taking a flight to Paris from Heathrow rather than schlepping into London for the train.

You have the same uncertainties of making a connection of course but can make a relatively airside connection at Heathrow and wait it out.

As someone who spends about three months a year in London, and have been doing so for a very long time, and have taken all forms of transportation from Heathrow....

First, I agree flying would be easiest but you have the same time allocations. You have no idea is your incoming flight will be on time.

If taking a rail type of transportation from Heathrow to St. Pancras, the Piccadilly line has no changes and less walking. When you get off at Kings Cross, you just follow signs to St. Pancras. It's literally across the street. from Kings Cross. This is also the least expensive way and will save you at least 25 GBP. Since you will be getting on at the first or secohd stop, depending on in which terminal you arrive, you should be able to get a seat. No changes, one hour--give or take a few minutes--and you are there.

You could take the Heathrow Express or Elizabeth line to Paddington and then a taxi. You will have to walk from the train to the taxi rank, and at that hour there will probably be a wait for a taxi. (Rush hour.). Lots of trains coming into Paddington and not just from Heathrow. The taxi will also take time due to traffic.

As stated, you have to be at the St. Pancras Eurostar check in area no less than 90 minutes before boarding. I'd suggest two hours as you have to check in, go through security and then go through French immigration. (When you arrive in Paris you just walk off the train and on your way.)

I agree that a time closer to noon would be safer. If you have time to kill, there are plenty of places to get breakfast or lunch.

With carry-on luggage, from a convenience standpoint, and if my goal is just to head to Paris, I would plan on flying out of Heathrow to Paris. An 11:00 AM flight would be OK, or if you want more buffer, there is a super cheap Vueling flight to Paris Orly at 2:35 PM, a short ride from central Paris on metro line 14.

If you take the Eurostar, then I would look into taking a slightly later train than necessary, e.g. the 3:31 PM, and treat yourself to a nice lunch in London.

Make sure you observe Eurostar's 90-minute early check-in, for immigration and customs before boarding. This is not optional, and has resulted in denied boarding despite arrival before train departure. I would pay extra for a flight direct to Paris from the US.