Piven beat out Kevin Dillon, his co-star on the celebrity-themed HBO show, as well as Jon Cryer from "Two and a Half Men," Neil Patrick Harris from "How I Met Your Mother" and Rainn Wilson from "The Office."
"To be a working actor is an unbelievable gift," Piven said upon collecting his award. "None of this is lost on me. Thank you so much."
Oprah Winfrey opened the 60th annual Emmy Awards show and introduced the ceremony's co-hosts Heidi Klum, Jeff Probst, Tom Bergeron, Howie Mandel and Ryan Seacrest -- all of whom were nominated for outstanding reality or competition series host this year.
"Baby Mama" co-stars Tina Fey and Amy Poehler, who appeared on "Saturday Night Live" last week as Sarah Palin and Hillary Clinton, then presented Piven with the first prize of the evening.
Jean Smart later won the Emmy for supporting actress in a comedy series for her work in the sit-com "Samantha Who?" The prize marked Smart's third non-consecutive Emmy win. She previously starred on the series "Designing Women."
Also nominated in the category this year were Poehler for "SNL," Holland Taylor for "Two and a Half Men," Vanessa Williams for "Ugly Betty" and Kristin Chenoweth for "Pushing Daisies."