He hasn't been since 2007, the year Congress beefed up those disclosure requirements.
The key distinction: Only employees who spend 20 percent or more of their work on lobbying or related activities have to register in Washington. Comcast says Cohen, an executive vice president, doesn't reach that threshold as he puts in 18-hour days spread across a wide array of responsibilities.
Still, when it comes to government affairs, the former chief of staff to then-Mayor Ed Rendell is the face of the growing cable and Internet giant.
Cohen testifies at hearings, visits White House policy summits, holds fund-raisers where President Obama praises him by name, and presses Comcast's case with top FCC officials.
But by not registering as a lobbyist, Cohen doesn't face limits on travel with lawmakers and doesn't have to file reports on his contributions to campaigns or lawmakers' pet foundations (though the campaign donations are made public through the Federal Election Commission). The longtime Democrat is also free from Obama's restrictions on appointing ex-lobbyists.
Like any entity that employs lobbyists - and Comcast had 11 in-house and more than 90 from outside firms last year - the Philadelphia-based company files reports showing what topics or bills it weighs in on.
But because Cohen isn't included, it's unclear what issues Comcast's point man takes on himself.
He is part of what ethics watchdogs say is a growing wave of heavy-hitting "shadow lobbyists."
The rules "are just riddled with loopholes that enable people to avoid registering as lobbyists but still engage in what, to most people, would look like lobbying," said Kathy Kiely of the nonpartisan Sunlight Foundation. "More and more lobbying has gone underground."
A request to interview Cohen for this article was answered by a Comcast spokeswoman. She said Cohen does not spend enough time on government affairs to trigger the reporting requirements.
"There are very clear legal definitions of what is a lobbyist, and we check them for all of our people who make government contacts every quarter and comply accordingly," said Sena Fitzmaurice, Comcast vice president of government communications.
Based in Philadelphia, not Washington, Cohen is responsible for government affairs, legal issues, communications, community investment, corporate real estate, and diversity, among other duties, Fitzmaurice said."David has a quite broad portfolio."
And while the New York Times and Washington Post have both referred to him as Comcast's chief lobbyist or chief of lobbying, the company fights such labeling.
Melissa Maxfield, once a political aide to Senate Majority Leader Tom Daschle, is Comcast's chief congressional lobbyist.
Cohen is one of many people who wield influence here without registering as lobbyists. Daschle, a Democrat, and former Republican House speaker Newt Gingrich are often cited as examples of movers who trade on their insider credentials but don't have to register.
"It's perfectly legal," Kiely said, but ethics groups scorn what they call the "Daschle exemption."
The lobbying law relies on self-reporting and has no serious enforcement behind it: if you estimate that you spend less than 20 percent of your work hours lobbying, you don't have to register.
It is akin to filing taxes with no risk of an audit, said Meredith McGehee, policy director of the Campaign Legal Center and part of an American Bar Association task force that urged revisions to lobbying rules in 2011.
Watchdog organizations and a lobbyist trade group, the Association of Government Relations Professionals, have called for decreasing or eliminating the 20 percent time requirement in order to cover more people who surpass other thresholds that require lobbyists to register, such as number of meetings with federal officials.
Lobbying, the ABA task force stressed, is a First Amendment right. Disclosure, McGehee said, "is to ensure the American people have a clear idea of both what was being spent and by whom to try to influence the outcome of activities in Washington."
Cohen, for a time, was a registered lobbyist: from 2002, the year he joined Comcast, to 2007.
In 2008, he stopped registering.
Fitzmaurice said Cohen had registered "out of an abundance of caution" but that an audit of his time showed he did not meet the time threshold.
He switched at a time when life was changing for Washington lobbyists. Congress had just passed the Honest Leadership and Open Government Act, strengthening disclosure requirements. Also, Obama in 2009 issued an executive order restricting his administration's hiring of ex-lobbyists.
For many, the downside of registering started to outweigh the cost of not doing so, McGehee said.
From 2007 to 2012, the number of registered lobbyists dropped by 25 percent, according to the nonpartisan Center for Responsive Politics.
Of course, whether or not Cohen is registered, his work is well known.
In 2013, he attended a meeting on violence in entertainment with Vice President Biden and media executives, raised money for Sen. Pat Toomey (R., Pa.) and testified at a Senate hearing on expanding broadband.
In November Cohen welcomed Obama to his West Mount Airy home for a fund-raiser aiding Senate Democrats. Obama lauded him and his wife with a riff on a Passover song of thanksgiving.
"If David and Rhonda had just hosted one of my first fund-raisers after I'd been elected to the United States Senate, that would have been enough. Dayenu," Obama joked, using the song's title and refrain, which is Hebrew for "it would have been enough."
"I have been here so much," the president added, "the only thing I haven't done in this house is have seder."
Less than a month later, on Dec. 9, Cohen and another Comcast official met with the chief of staff, senior counsel and chief counsel to FCC Chairman Tom Wheeler, a recent Obama appointee. They talked about a program to expand Internet service to schools and Cohen pushed to open more airwaves for WiFi, according to Comcast's FCC disclosures.
Cohen, along with two company lobbyists, also sat with FCC Commissioner Michael O'Rielly to argue their case in a long-running dispute over the terms of how Comcast, under the conditions of its NBCU merger, must supply programming to online video companies.The two meetings took up about 90 minutes of a day that began early in Philadelphia and ended with Cohen reviewing work in his hotel room until around 1 a.m., Fitzmaurice wrote in an e-mail.
She said they were the only meetings or calls with government officials Cohen held while working seven days that week.