The first signs of manga in the western world came in through magazines - Much like how manga first came to Japan before being compiled into full books.
Occasionally publications would include smaller works or partial translations of a larger manga. Notably (as @krazr has pointed out) Concerned Theatre Japan's second volume, 2nd issue 1971 is a manga special with 3 short works translated.
This issue features Sakura Gaho (Eng. "Sakura Illustrated" - excerpt) Akame (Eng. "Red Eyes" - Part One)," and "Nejishiki" (Eng. "The Stopcock" - Short work)
The first fully independent manga series that was translated into English and published was Barefoot Gen in 1979.
Barefoot Gen is an epic 10 volume manga of the story of a boy during the Atom bombing of Hiroshima. The piece takes many sections from Keiji Nakazawa's own experiences as a Hiroshima survivor. Feeling the work should be spread for the largest impact, a pacifist translation charity Project Gen starting working on translating it into English in 1976.
The first two translations were published in 1979. Unfortunately, it wasn't as successful as first thought and no more volumes were published. It was only in 2004 that the full 10 volumes eventually got fully released (by Last Gasp).
Prior to this, there was a comic adaption of Astroboy redrawn by Americans in 1965. Barefoot Gen retained the original artwork, so is usually considered the first.
Astroboy was the first anime series to be brought overseas, in the same year as it was televised nationally (1963) and much like today - it was seen a definite secondary market to Japan. An earlier 3-part work, Three Tales was released 2 years beforehand in 1961 (A year after it aired in Japan)
Hakujaden was one the first Japanese animation films to make it to the US under the title Panda and the Magic Serpent (and also the first anime movie) in 1961 along with two other titles - Magic Boy (premièred a month earlier) and Alakazam the Great