I did some research and came up with these first ascents. My sources were the Watermans amazing Forest and Crag and my Thru Hikers Guide to America. These two books are built on many, many sources.
1. Mt Washington, NH: Darby Field and two Indian guides. 1692. (Ibid)
2. Mt Marcy, NY: Emmons, Hall, Redfield, Henderson, Torrey, Ingham, Cheney, and Holt. August 5, 1837, 10:00 am. (Ibid)
3. Mt Mansfield, VT: Ira Allen, Baker, and survey party. Summer 1772. They actually reached the "Adam's Apple," which is 4,060 feet, but probably climbed to the true summit for a better view of Allen's recently acquired land. (Ibid)
4. Mt Greylock, MA: Probably Timothy Dwight. 1799. (Waterman)
5. Mt Frissell, CT: Perhaps Timothy Dwight. 1781. He was reported to be on Mt Everett to the north of the CT highpoint. (Waterman)
6. Mt Katahdin, ME: Charles Turner Jr. and survey party. August 13, 1804, 5:00 pm. (Waterman)
Concerning Northeast peaks, there is little chance that Indians reached the tops prior to hunters, climbers, and surveyors (the primary first ascentionists of the Northeasts 3,000 and 4,000-footers). The main reason being that there was more to do in life than hike up big peaks. Nomadic movement, warfare, family obligations, hunting, and gathering being some core activities.
"The Native Americans who were in this region before the European settlers reportedly had little to do with the higher mountains. No trace of trails to summits, no cairns, or any other evidence of climbing the bigger peaks was found. For the Indians these mountains had no practical uses. They could not be farmed. The upper elevations were rarely frequented by game. Routes of travel usually avoided ups and downs where possible and clearly kept away from thick, stunted, coniferous forests of higher reaches." (Waterman).
7. Humphreys Pk, AZ: The first recorded ascent of the peak was prior to 1900, but its likely Native Americans reached the top well before that time, since the Sinugua Indian culture lived in present day Flagstaff prior to the year 1000. Luckily no one attempted to climb the peak way back in time, when the San Francisco Peaks comprised one huge mountain that might have been 20,000 feet tall. (Schlimmer)
8. Mt Whitney, CA: This dramatic granite peak, described by the early Sierra Nevada mountaineer Clarence King as looking like the prow of an ocean steamer, received its first recorded ascent in 1873 by three fishermen, of all people. The first ski descent from the exact summit was made by Paul Arthur and Larry Yout in 1958. (Schlimmer)
9. Mt Mitchell, NC: Mitchell may have been the first person to summit Mount Mitchell in 1835 after viewing it from nearby Big Butt (5,940 feet). The good doctor had his doubters though, and in 1855 an intense controversy ensued between Dr. Mitchell and Senator Thomas Clingman (who has two southern high peaks named for him) over whether Mitchell did in fact summit the highest peak east of the Mississippi. In June 1857, still in the heat of dispute while on a third expedition to the summit, Mitchell unfortunately fell to his death at a forty-foot waterfall, now called Mitchell Falls (with no pun intended). (Schlimmer)
10. Mt Rainier, WA: Mt Rainier was possibly first summited in 1855, but many regard 1870 as the year of the first successful ascent. Mount Rainier's first winter ascent was completed in 1922 by Jacques Bergues and Jean & Jacques Landry. (Schlimmer)
I hope this helps you out, Steve and all others. Id be very interested to hear if someone is attempting to assemble a list of first recorded ascents and first probable ascents for the 50 highpoints. It's all very interesting to me at least. E. Schlimmer