He previously was a co-host of ESPN talk show First Take alongside Stephen A. Smith and Molly Qerim.

Given the medium, the show was quite simple but nevertheless caught the attention of the boxing community, including a young Mike Tyson.

In the late 1990s after graduating from Columbia University with a degree in history, Kellerman was hired as an analyst on ESPN's boxing series Friday Night Fights where he met future radio partner Brian Kenny.

He was originally hired for the network's Boxing After Dark telecasts, working alongside Fran Charles and Lennox Lewis.

In 2007, Kellerman moved up to the HBO World Championship Boxing main team alongside Jim Lampley, Harold Lederman, and a rotating guest analyst, usually Roy Jones Jr. or Emanuel Steward.

"[3] Kellerman was announced in December 2010 as the new midday host at ESPN Radio station KSPN in Los Angeles, alongside former NFL player co-host Marcellus Wiley, replacing LA Sports Live with Andrew Siciliano and former NBA player Mychal Thompson.

[6] The Guardian stated that his line of questioning was, "interpreted as patronizing" as his wording to Pacquiao was "You thought you won today, Why?"

Kellerman replaced Skip Bayless on ESPN's First Take as co-analyst alongside Stephen A. Smith and host Molly Qerim, effective July 25, 2016.

However, in contrast to his color-commentary role with HBO, for ESPN he presided over network coverage from the broadcast desk rather than at ringside.

[9] Kellerman departed from First Take in September 2021 after Stephen A. Smith requested his removal, citing a lack of chemistry necessary for the show’s long-term success.

He graduated from New York City's PS 41 in Greenwich Village, Hunter College High School in 1991 and, later, Columbia University in 1998.

[17] A hip hop fan, evidenced by his intro music on his ESPN Radio show, Kellerman's favorite artists include the Wu-Tang Clan, having stated on his radio show that Raekwon's Only Built 4 Cuban Linx... is one of his favorite albums.

Kellerman has a small permanent scar on the left side of his mouth from a childhood accident with an electrical outlet.