The origin is as given on this site.
Chelsea probably became a girl's name because it rhymes with
Elsie and
Kelsey, and because the district named
Chelsea in
London has some artistic and "bohemian" associations for many Australians and Americans.
Chelsea seems to have been first used as a girl's name in Australia. 1969, the first year it was on the Social Security top 1000 list for girls in the USA, was the same year
Joni Mitchell's song
Chelsea Morning was a hit, though that may be coincidental. There was a boom in use of the name in the USA after the 1981 film
On Golden Pond, which featured a character named
Chelsea played by
Jane Fonda, was released. The peak year for
Chelsea's popularity in the USA was 1992, the same year you were born. :)