Illinois shot 1-under par, winning by eight shots over solo second-place finisher Arizona State, who shot 7-over. The Fighting Illini had their best round in round two, shooting a team score of 281, better than any other team round score by five rounds.
Washington finished in solo third place at 10-over, Arizona finished in solo fourth place at 12-over, and Duke finished in solo fifth place at 16-over.
With the win, Summerhays gets an exemption into the Fortinet Championship this week in Napa Valley.
Summerhays finished 5-under after rounds of 72-67-72.
Herendeen also finished 5-under after rounds of 74-64-73 but lost in the playoff to Summerhays to finish in solo second place.
Herendeen grew up just minutes from Sahalee Country Club in Bellevue, Wash.
Filip Jakubcik of Arizona finished in solo third place at 1-under, and Chuan-Tai Lin of Washington finished in solo fourth place at even-par.
Summerhays went 2-1-1 in helping USA win another Walker Cup earlier this month, and the two-time All-American has now won two of his past five events, as he also captured the 2023 ASU/Thunderbird Intercollegiate in April.
It marked his second career win and his fifth top-five finish as he tied for fourth at the 2023 Valspar Invitational, the 2023 National Intercollegiate, and tied for fifth at the 2023 Cabo Collegiate.