Dickinson's knee injury prevented him from moving to Leeds during the early stages of the Football League's emergency loan window.
United lined up an emergency deal for Dickinson after originally failing to sign him for the rest of the League One campaign on the last day of the January transfer window. The club were unable to officially complete the switch before the 5pm deadline on February 2.
A subsequent attempt to revive the move through the Football League's emergency loan system was hindered by the forward's loss of fitness, but he finally linked up with United's squad this morning and was in contention to make his debut against Swindon Town at Elland Road.
Football League clubs are permitted to sign players on loan outside FIFA's transfer windows for a maximum of 93 days and, having failed to secure him at the first attempt in January, United's intention was always to bring in Dickinson after February 21 to ensure that his contract would run for long enough to make him available for the League One play-offs should Leeds qualify.
The 23-year-old's loan from Pride Park is currently due to expire on May 2, when Leeds complete their 46th and final League One fixture at home to Northampton Town, but his stay will be automatically extended by County if United find themselves involved in the play-offs for the second season running.
Dickinson is on the fringes of the squad at Derby, who he joined from Stockport County for £750,000 last summer, and he has already completed loan spells with Huddersfield Town and Blackpool this season.
He was part of Blackpool's squad when Simon Grayson left Bloomfield Road to become manager of Leeds in December, and despite failing to feature for the first team at Pride Park this term, he has scored 10 goals in 20 appearances during his time away from Derby.