The media regulator found that TalkSport had "seriously breached the due impartiality rules at the time of an election", when Whale criticised Ken Livingstone and "directly encouraged listeners to vote for Johnson".
Ofcom said: "Breaches of this nature have the potential to cause considerable harm to the democratic process by conferring an unfair electoral advantage on a particular candidate."
Whale made the remarks during his late-night talk show on March 20 this year and was subsequently sacked by TalkSport after 12 years at the station.
A TalkSport spokesman said at the time that Whale had been "guilty of a gross error of judgment, which we found to be totally unacceptable".
Ofcom previously rapped TalkSport over a similar case last year.
In November 2007 it ruled of the broadcasting code on impartiality in politics when phone-in host and MP George Galloway criticised a rival candidate for the constituency of Poplar and Limehouse, Jim Fitzpatrick.
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