I dunno how consistently “dramatic” The District was - it seemed to me that Nelson’s character was always going off on some goofy tangent before things got all serious and dark and preachy and it’s about the kids, dammit!
I nominate Maura Tierney, who got her big break on NewsRadio (admittedly as one of the more straitlaced characters) before moving on to ER.
Also, Katey Sagal, his co-star from Married With Children, who is in the drama Sons of Anarchy.
Judd Hirsch (Taxi/Numb3rs)
Rick Schroeder (Silver Spoons/NYPD Blue)
Josh Charles (Sports Night/The Good Wife)
Chris Noth (Law & Order/Sex & the City/The Good Wife)
Bill Bixby (The Courtship of Eddie’s Father/The Incredible Hulk)
Brian Keith (Family Affair/Hardcastle & McCormick)
Gerald McRaney (Simon & Simon/Major Dad/Promised Land)
Jim Sikking (Hill Street Blues/Doogie Howser, M.D.)
Susan St. James (McMillan & Wife/Kate & Allie)
David Duchovny - the X-Files and Californication. (He was in other stuff too, of course).
Garret Dillahunt played evil men in three separate crime procedural dramas that I’ve seen - White Collar, Burn Notice and Life, the latter two roles being only for a few episodes, but the characters were significant - and was a one-off guest actor in lots of others too (I don’t know if he was always a bad guy, but I suspect he was). He was also the main terminator in both seasons of The Sarah Connor Chronicles. He was excellent at being a sociopath.
And then he became the loveable Dad (and Grandad) on Raising Hope.
Alyson Hannigan - Buffy, the Vampire Slayer and How I Met Your Mother (it occurs to me that if you just knew the titles and not the shows, you’d assume Buffy, the Vampire Slayer was a sitcom and How I Met Your Mother was a drama).
Enrico Colantoni - Just Shoot Me and Veronica Mars
Also, Charlie Hunnam, her co-star from Sons of Anarchy, who was in the short-lived sitcom Undeclared (and also in the drama Queer as Folk).
We could do this all day.
Tom Selleck eventually followed up his Emmy win for MAGNUM PI with an Emmy nomination for FRIENDS.
Also, Jay Baruchel, his co-star from Undeclared, who was in the short-lived drama Just Legal.
I am surprised no one has yet mentioned Larry Hagman (I Dream of Jeannie/Dallas).
How soon the pioneers are forgotten.
Jack Klugman: 114 episodes of The Odd Couple and 148 of Quincy M.E.
Kind of a stretch to call being in one drama that was cancelled after one season and not getting a role in another drama being “successful in dramas”
Does Susan Dey qualify? The Partridge Family and L.A. Law
Actually, looking at wikipedia, poor O’Neil has been in like five failed TV dramas that either went one season or never got passed the pilot. I doubt they failed because of him, but he seems to be something of a kiss-of-death for dramatic TV.
Ron Howard had two hit shows. Happy Days and Andy Griffith.
Don Knotts also. Three’s company and Andy Griffith.
John Ritter is another with more than one show.
Jason Lee in My Name Is Earl, and now with Memphis Beat.
But these are all comedies.
Alan Alda - MASH and The West Wing
Candice Bergen - Murphy Brown and Boston Legal
Janeane Garofalo - The Larry Sanders Show and Criminal Minds: Suspect Behavior
Thomas Gibson - Criminal Minds and Dharma and Greg
John Larroquette - Black Sheep Squadron and Night Court
Matthew Perry - Friends and Studio 60 on the Sunset Strip
Robert Lyndsay, started his career in sitcom Citizen Smith, drama performances include roles in Hornblower, Oliver and much Shakespeare, has played the role of dentist Ben Harper in sitcom My family for the past 12 years.
His screen wife in My Family, Zoe Wanamaker, is mailnly known for her dramatic roles, appearing in Dicken’s adaptations, Dr Who and many, many others.
Stephen Fry, of course, has done everything.
James “Frickin’ Chuck Norris Calls Me Sir” Garner.