The 32 year old striker made 133 appearances for City scoring 31 goals and has also played for Leeds United, Middlesbrough, Palace, Millwalll, Derby, Aberdeen, Boston, Dunfermline and Harrogate Town.
He was recently the Legends Lounge guest for City's home game against Middlesbrough.
Darlington are currently bottom of League Two and Whelan has joined on a non-contract basis which can be terminated at any time.