Thompson finished with a two-day total of 6-over-par 150 to earn medalist honors for the first time at the Senior Women's Amateur.
'It's not something I was trying to be,' said the three-time defending champion from Sewickley, Pa. 'It's really nice being medalist, and I don't mind getting up early.'
The 53-year-old will follow the tradition of playing in the first match of match play on Monday. Thompson will face Marilyn Nolan-Johnson in the first round.
Thompson picked up her first birdie of the day at the par-3 second but ran into trouble with a bogey at the fifth and faltered again with back-to-back bogeys from the ninth.
She responded well with a birdie at the 11th and parred the remaining holes.
Toni Wiesner of Fort Worth, Texas, finished two strokes behind Thompson at 8-over-par 152 after a round of 77 on Sunday. She was followed by Marsha Butler of San Diego, Calif.
Thompson, a six-time USGA champion, has an 18-0 record at the Senior Women's Amateur. She is one win away from tying Glenna Collett Vare and Alexa Sterling for most consecutive matches won in a single championship.
'I'm just amazed at it all,' said Thompson.
Nolan-Johnson, 50, of Marietta, Ga., earned her way into match play through a late playoff. Eleonar Walker of Greensboro, N.C., and Dianna Yelovich of San Pedro, Calif., also qualified through the playoff.
The U.S. Senior Women's Amateur is one of 13 national championships conducted by the USGA, 10 of which are strictly for amateurs. The tournament was first played in 1962 and is open to women golfers over the age of 50.