In 1967 he became a student of Masachika Sugimura before moving to Tokyo, where he worked part-time at an artist's material shop and later as an illustrator and a designer for an advertising company.
In 1988 he produced an official portrait of U.S. President Ronald Reagan and started a series of work about golf in collaboration with Jack Nicklaus.
He donated all sales of his piece, "Fireworks," to the International Red Cross Society for victims' relief of the San Francisco earthquake.
In the 1990s, Yamagata created official art works for a celebration of 200 years of emigration to America (1990), the Freedom Campaign in Berlin (1990), the 500th anniversary of Columbus's first visit to the New World (1991), the 3rd IAAF World Championships in Athletics, Osaka (1991), the Barcelona Olympics (1992), Kyoto 1200 year celebration (1992) and the Atlanta Olympics (1995).
He exhibited his painting on a vintage Mercedes-Benz car as a description of beauty of the nature, "Earthly Paradise," at Los Angeles Municipal Art Gallery.
Glen Ginsberg wrote a description of the car as "Hiro Yamagata's spirit, the automobile of the 20th century" for the exhibition.
The Earthly Paradise exhibition was held in Hakone, Venice, Monte Carlo, Montecatini, and Tirino in 1995, Stockholm in 1996, Vienna in 1997 and Rome in 1998.
He showed a work from "Earthly Paradise" incorporated with laser for the décor of the Academy Awards Governor's Ball at the Oscars.
He also produced a movie which is related to the Beat Generation, a major feature documentary, The Source, and presented at Museum of Contemporary Art in Los Angeles and New York City.
Also, a PlayStation game which Hideki Tougi took charge of its music part, "YAMAGATA Digital Museum," was released.
In 2000, Yamagata participated in a laser installation group exhibition, "An Active Life," at Contemporary Arts Center in Cincinnati, Ohio.
His piece in this exhibition was held indoors and made by laser reflection of innumerable cubes hanging from the ceiling of two huge cubic structures which was surrounded by holograms.
Later on, Yamagata held a laser installation exhibition, "Super Nova 3" at COSI Columbus Science Museum in Ohio.
Also, he participated in an event, the "300th anniversary of Sankt-Peterburg municipal government, a Night of Sound and light," to show his laser installation.
In 2005, Yamagata proposed a large-scale holographic recreation of the destroyed statue of the Buddha at the Archaeological Remains of the Bamiyan Valley, Afghanistan.
The proposal included using solar energy saved during the day to power a laser installation after sunset, and was projected to cost $60,000,000.