Can I buy Shinkansen ticket on the spot?

If I reach Shinagawa station at 6:21am, can I buy shinkansen ticket and seat reservation to Kyoto on the spot?

What is the operating hour for JR Ticket Offices (Midori-no-madoguchi)?

Thanks

Yes, if there are vacant seats, you can make a reservation on the spot.

What is the operating hour for JR Ticket Offices (Midori-no-madoguchi)?

At Shinagawa Station it is from 5:30 to 23:00.

Thank you.

unless it's a holiday it's almost never an issue finding a seat. Whenever I get up and head to the station, that's when get the ticket. I like to have flexible plans.

BTW
If you find you can not make a reservation because all reserved seats are taken, or for instance the seats on the right side are occupied (this is the side from where you can see Mt Fuji), you can always try to get a seat in the non-reservation cars. The earlier you can get in line on the platform the more lucky you can get. For me this worked a couple times.
Also because the trains bound fot Kyoto start at Tokyo station you can be more lucky to get a seat in the non-reserved cars at this station.
Have a nice trip
B. Slager

If I reach Shinagawa station at 6:21am, can I buy shinkansen ticket and seat reservation to Kyoto on the spot?

What date are you travelling?
What airport do you arrive at? What date/time?
What airport do you depart from? What date/time?
What is the rest of your itinerary?

If you are happy to pay full fare for your ticket then as already explained you can purchase your ticket at Shinagawa.

If you arrive late the night before or early in the morning at Haneda and wish to use your JR Pass to get a seat to Kyoto then you can't do so at 6:21am as you won't be able to exchange your JR Pass in time.

It is possible that a JR Pass won't pay off for your itinerary but it may be worth investigating, depending on your answers to the above questions.

Enjoy your trip!