Plex attended twelve semesters, expanding his studies in different fields, was offered a department chair upon his graduation and returned years later for guest lectures.
Barnhart was very active in Manhattan during the late 80s and the 90s; within his first month of arriving, he was recording and touring with Melanie, and penning charts for drummer Steve Ferone.
Roger Talkov, a Hit Factory engineer, introduced him to Keith Richards, Brian Wilson, Paul Simon, Phil Ramone, Elliot Easton, and Kim Turner (Sting's manager) all of which Plex provide various services.
During this time Plex performed live with Chaka Khan, John Entwistle, Mitch Mitchell, Steve Winwood, Richie Sambora, Billy Squier, Phoebe Snow, All-4-One, along with many of NYC A-list musicians.
It was at the famed China Club where Plex caught the interest of David Bowie’s musical director and guitarist Carlos Alomar who quickly pulled him in for the Debbie Gibson world tour.
The tour got cut short due to poor record sales and the Gulf War which broke out the same night Plex performed in front of 200,000 fans in Rio de Janeiro, Brazil.
He played keyboards in two MTV videos: Sweet Sensation's "Sincerely Yours" in the Apollo Theater, and Debbie Gibson's "This So-called Miracle" in a Hollywood studio.
In 1990, Barnhart began composing ads for Macy's National with producer John Wonderling (Bay City Rollers and Allan Toussaint), a working relationship that lasted over 12 years.
[citation needed] In 2000, Barnhart composed the music for three songs by Lebanese singer-songwriter, Lydia Canaan (known as the first "rock star" of the Middle East).
Earlier that decade, Barnhart was invited to submit cues from his personal library for syndicated TV shows: America's Most Wanted, Greatest American Hero, 21 Jump Street, Hunter, and The Commish.