Underwood first appeared in Playboy in its Girls of the Pac-10 pictorial in the October 2005 issue. She was featured on the cover, holding a football and clad only in a body painted rendering of an Oregon State Beavers football jersey and matching bikini briefs. She next appeared in the magazine in July 2006, as the Playmate of the Month. When she became Playmate of the Year in June 2007, her prize included a Mini Cooper convertible. She was the first Miss July to receive the title, although she herself said, “I didn’t think I was pretty enough”. In March 2008, Playboy magazine ranked her at #25 on their “25 Hottest Playboy Celebrities” list.
Underwood appeared in episode #2 of the E! network’s series The Girls Next Door, as a Playmate hoping to have her test shoot taken at Playboy Studio West. Subsequently, Playboy featured her as Miss July 2006. She appeared in several more episodes of The Girls Next Door.
On June 7, 2007, a group of Oregon State University women’s studies students created a poster celebrating Underwood’s selection as Playboy’s 2007 Playmate of the Year and posted it outside Bexel Hall, where it remained in place for only a few hours before Oregon State University staff took it down. The poster embraced the theme and the style of the “Achievement” posters that line Campus Way at the university. The poster read, “Sara Jean: First OSU Beaver Playmate of the Year, Playboy June ’07, OSU Centerfold, People, Ideas, Innovation.” The mention of her as the “first” Playmate of the Year from OSU was actually incorrect; she was preceded by Jodi Ann Paterson, the Playmate of the Year for 2000.
Underwood is credited with discovering 2009 Playmate of the Year Ida Ljungqvist while shopping on Rodeo Drive in Beverly Hills, California.