The 22-year-old linked up with the capital outfit in July on a loan deal which ran through to the end of the MLS campaign.
He impressed enough during his short time back in the States to convince DC United that he was worth keeping.
A switch has now been completed, with Doyle heading home after three years at Derby which saw him make 23 senior appearances - with most of them coming in the 2010/11 campaign.
He posted on Twitter: "Buzzing to start this new phase of my life by making my move to DC United permanent. Thanks to Derby for everything the last three years."
DC United are equally delighted to get a deal pushed through for a player they hope will have a big part to play in their future.
Scouting coordinator Kurt Morsink told the club's official website: "Conor is a very talented, young forward who will continue to improve.
"His ability to get into good spots and anticipate service is very rare for his age, and makes him a unique forward."