After struggling mightily with his control in his first three seasons, the 6-foot, 175-pound southpaw transformed into the Tigers' ace this year, reviving his Draft stock in the process.
He lowered his walk rate from 13.69 to 2.79 per nine innings and posted a 2.33 ERA with 77 strikeouts in 58 frames.
In May, Fagan was named to the All-Ivy League First Team.
"He obviously had a strong year this year, but you're talking about a lefty that pitches with an average fastball that for his career has had more strikeouts than walks," A's assistant director of scouting Michael Holmes said.
The only other player from Princeton ever drafted by the A's was Drew Stratton in 1986.
The Draft concludes Saturday, with exclusive coverage of Rounds 11-40 beginning on MLB.com at 10 a.m. PT.
Aaron Leibowitz is an associate reporter for MLB.com.