Seibert began his media career in college radio at Columbia University’s WKCR-FM in 1969. While at Columbia he started his first company, Oblivion Records with partner Tom Pomposello, releasing LP’s by Mississippi Fred McDowell (Live in New York) and Joe Lee Wilson. Simultaneously, he produced several dozen jazz and blues albums for independent companies such as Muse Records, JCOA Records (Jazz Composer’s Orchestra), and Birth Records (owned by instrumentalist/composer Gunter Hampel). Seibert was an early employee of New Music Distribution Service, a non-profit distributor of musician owned record company started by composers Carla Bley and Michael Mantler, before going on the road with Bley’s big band as sound engineer and road manager. After a late 1970s stint with media promotion innovator Dale Pon at New York’s WHN Radio, Seibert began his work at Warner-Amex Satellite Entertainment in 1980.

In 1984, with partner Alan Goodman at Fred/Alan Inc., Seibert successfully overhauled the then-floundering children’s cable channel Nickelodeon, moving it from worst to first in the ratings in six months.

From 1992 until 1996, as the last president of the Hanna-Barbera cartoon studio, Seibert was able to reinvigorate the company’s creative reputation with the establishment of the animation incubator What a Cartoon!. Modeled on the Golden Age of mid-20th century cartoons, the 48 short films from creators around the world, Hanna-Barbera was able to launch seven hit series after a dry spell since the launch of the Smurfs in 1981 for NBC. The shows included Genndy Tartakovsky’s Dexter’s Laboratory, David Feiss’ Cow and Chicken and I Am Weasel, Van Partible’s Johnny Bravo, John R. Dilworth’s Courage the Cowardly Dog, and Craig McCraken’s The Powerpuff Girls.

After starting Frederator Studios in 1997, Seibert brought together a group of investors in a failed attempt to save the troubled underground/alternative comics publisher Kitchen Sink Press.

In March 1999, MTV Networks CEO Tom Freston tapped Seibert to become the first president of the new MTV Networks Online, soon to split into MTV Interactive (The MTVi Group) and Nick.com.

In 2004 David Karp interned at Frederator Studios at its first New York City location, and built the company’s first blogging platform. In 2007 he launched Tumblr from a rented desk at Frederator Studios’ Park Avenue South offices, with chief engineer Marco Arment. Seibert was one of Tumblr’s first bloggers, an angel investor in the company, and served on its board before its acquisition.

Building on this new media success, in 2007 Seibert conceived and founded Next New Networks (with Emil Rensing, Herb Scannell, Tim Shey, and Jed Simmons), the leading online television company, with over 2 billion all time video views and over 200 million views every month (as of 2010). Along with their affiliated Indy Mogul, Barely Political, Channel Frederator and several other networks, the company’s superdistribution has allowed it to become among the most widely distributed video in the world, and to become YouTube’s top professional content provider. By the end of 2010, Next New Networks had the globe’s top two videos viewed on YouTube. In March 2011, Next New Networks was acquired by YouTube.

On February 21, 2012, Fred Seibert launched Cartoon Hangover, a channel on YouTube which consists of various animated shorts and series. Cartoon Hangover gained a much larger audience with the revival of Bravest Warriors by Pendleton Ward on November 8, 2012 which originally aired as a pilot on Fred Seibert’s Random! Cartoons on Nicktoons Network in 2009. In 2014, Channel Frederator was revived as a multi-channel network focused entirely on animation, signing one of YouTube’s biggest animation channels, Simon’s Cat. By September 2014, the network was distributing 688 channels, with over 65 million monthly views and 10.5 million subscribers, and by 2017 announced it had reached 1 billion monthly views on YouTube.

Seibert and his Frederator Networks partnered with John Borthwick and Betaworks; Jonathan Miller, Jason Ostheimer, Shari Redstone; and entrepreneur Yoel Flohr to form Thirty Labs in 2014, a startup studio based in New York City to develop and invest in video based technology businesses Seibert is CEO and Flohr, COO. The company shuttered in 2015.