In September 2013, he lost the primary election for the Democratic nomination for the New York City Council seat for the 5th District to attorney Ben Kallos.

He worked as an aide to Senator Chuck Schumer, Congresswoman Carolyn B. Maloney and City Comptroller William C. Thompson before being elected to the Assembly in 2007.

[5] Following the appointment of Assemblyman Alexander "Pete" Grannis as Commissioner of New York State's Department of Environment Conservation early in 2007, Kellner was selected as the Democratic nominee for the special election held to fill the vacancy.

The law was named after Elle Vandenberghe, a three-year-old Upper East Side girl living in Kellner's district who was seriously injured after being struck by a motorist who was illegally backing through an intersection against a red light to find a parking space.

The law is named in memory of Oreo, a dog who survived abuse at the hands of her former owner, recovered from her injuries, but was then euthanized by the ASPCA in New York City, despite the offer of a No Kill sanctuary to guarantee her lifetime care.

[10] In early 2013, Kellner announced his intentions to run for the New York City Council seat in District 5 being vacated by Jessica Lappin.

Kellner apologized in a statement to the Times, saying, "Over four years ago, for a few weeks while I was still single, I exchanged instant messages with a female member of my staff that were flirtatious.

[12] Following the media reports, several elected officials withdrew their endorsement of Kellner in the City Council race, including Lappin, Manhattan Borough President Scott Stringer, and two state senators (Jose Serrano and Brad Hoylman).

"[24] He was one of six openly LGBT members of the New York Legislature, alongside Assemblymembers Deborah Glick, Daniel O'Donnell, Matthew Titone and Harry Bronson, as well as Senator Brad Hoylman.