Mark D. Nielsen is an American business executive, former elected official, and attorney.

Frontier is America's fourth largest landline telecom company providing data and voice services in 25 states.

Nielsen started his legal career in 1990 as an associate lawyer at the Hartford law firm of Murtha, Cullina, Richter & Pinney, concentrating on federal and state litigation.

Nielsen's public roles have included state representative in Connecticut (1992-1994), state senator in Connecticut (1994-1998), and staff member to Mitt Romney when Romney was Governor of Massachusetts.

[8] In his memoir, In My Time, Vice President Dick Cheney acknowledges that he was initially angered by Nielsen’s favorable comments about Cheney's opponent in the 2000 election, Senator Joe Lieberman, but that, upon reflection, he couldn’t disagree with what Nielsen had said about Lieberman.