The first episode broadcast in the United States ("Battle of the Cameras") actually features two opening credit sequences. The first is a brief, 10-second introduction featuring a few bars of "Secret Agent Man" and a credit for Patrick McGoohan (running roughly the same length as the original UK credits). This is followed by the teaser, and then the regular credits. In all future US broadcasts, the pre-teaser credit sequence was dropped.
The opening credits for the original British version of the series are only about 10 seconds long -- one of the shortest credit sequences in TV history (among shows with opening credits). The American version of the opening credits is some 3 times as long.