McClendon finished with 11 kills for Penn State, which is 6-3 overall in NCAA championship matches. Megan Courtney had 20 digs.
Dominique Thompson had 16 kills for Wisconsin and 5-foot-7 outside hitter Deme Morales added 15. The Badgers were making their first appearance in the championship match since 2000, when they fell to Nebraska in five sets.
Wisconsin won the third set and strung together a key four-point run to go up 17-15 in the fourth. Ellen Chapman's ace made it 22-19, but a missed serve by Annemarie Hickey hurt the Badgers. Micha Hancock's ace for Penn State narrowed it to 23-22, and Slay's kill down the middle evened it.
Hancock had another ace before McClendon's soft hit fell unreturned for the victory.
Penn State, led by national coach of the year Russ Rose, swept Washington in the semifinals. Surprising Wisconsin downed top-ranked Texas, the defending champion, in straight sets to advance to the final.
The Badgers came out with determination, going up 5-3 before the Nittany Lions, with a distinct size advantage, evened it at 9 on McClendon's cross-court kill before pulling in front on 6-foot-6 Slay's swing.
A five-point Penn-State run made it 20-15. Ellen Chapman's tap over the net for Wisconsin held off set point until Scott's kill to give the Nittany Lions 1-0 advantage.
The Nittany Lions beat Wisconsin twice in straight sets during the regular season.