Quintanilla attended the University of Colorado at Boulder where in 1993 he received a Bachelor of Arts degree in political science. He interned at Westword Magazine in Denver, CO under Patricia Calhoun the editor-in-chief.
From 1991 to 1993, he was a reporter and columnist for the Boulder Daily Camera in Boulder, and prior to that, he spent a summer as an editorial assistant for National Public Radio in Washington D.C.
From 1994 to 1999, Quintanilla served as a staff reporter for The Wall Street Journal where he wrote full-time for the newspaper's Chicago, Illinois, bureau, covering airlines, manufacturing and economic issues. He also wrote a weekly column on workplace issues and on-the-job trends for the newspaper's front page.
From 1999 to 2002, he served as correspondent for several CNBC programs including Business Center as well was a sometime special correspondent for Fox News Channel program Fox X-press. Prior to joining NBC, Quintanilla served as co-anchor for CNBC's early-morning program, Wake Up Call.
Quintanilla was assigned to cover the 2006 Israel–Lebanon conflict. In 2007, he traveled to China to cover McDonald's efforts in the country for CNBC's documentary Big Mac: Inside the McDonald's Empire.
He, along with his network, CNBC, was berated by Jon Stewart in the aftermath of the 2008 global financial crisis for failing to predict the downturn and ask tough questions of Wall Street executives. Quintanilla had once asked Allen Stanford, later pronounced the orchestrator of a "massive Ponzi scheme", how it felt to be a billionaire on his show – instead of "tougher" questions. This drew the ire of Stewart and other media commentators.
From 2010 until June 2015, Quintanilla substitute anchored weekday and weekend editions of "NBC Nightly News", covering when hosts Brian Williams and Lester Holt, respectively, were on assignment or away. He also substitute co-hosted the Today Show on weekends for Holt, and occasionally for Matt Lauer on the weekday program.
In July 2011, Quintanilla left CNBC's Squawk Box (which he had co-anchored since December 19, 2005) to join the new Squawk on the Street anchor team at the NYSE.
In June 2014, Quintanilla also joined HBO Real Sports as a correspondent. He presented a story on Stephon Marbury in January 2015.
On October 28, 2015, Quintanilla was one of CNBC's moderators of the third Republican presidential candidates debate at the University of Colorado Boulder. He and his CNBC co moderators were heavily criticized for being ill prepared and rude to the candidates.
Quintanilla won an Emmy Award, an Edward R. Murrow Award and a Peabody Award for his coverage of Hurricane Katrina in 2005.