Johnson was Northwestern's top returning receiver with 176 career receiving yards.
There were 14 Power Four commitments over the weekend, including Northwestern pledge, CB Joshua Sims.
Northwestern offered WR Justin Johnson, a 2027 prospect in their backyard at Evanston Township High School.
Holy Cross guard Max Green has committed to Northwestern with three years of eligibility remaining.
Tenn. CB Joshua Sims is the second member of the Wildcats' Class of 2026 after announcing his commitment on Saturday.