Christensen was born and raised in Culver City, California. Christensen began his studies at Santa Monica City College. He later went on to attend UCLA. He then moved to Utah to finish his higher education at Brigham Young University (BYU). Christensen only started oil painting after he began studying at BYU. He would eventually earn a master of arts degree from BYU.
After college Christensen began his career as a free-lance illustrator. He was also a junior high school art instuctor in California for a time.
Christensen taught art at BYU from 1976 until 1997.
He has had numerous showings of his work throughout the US and has been commissioned by media companies to create artwork for their publications, such as Time-Life Books and Omni.
His artwork has been featured on the cover of Leading Edge issue #41, winning him the Chesley Award for cover artwork in 2002. Christensen's work has appeared in the American Illustration Annual and Japan's Outstanding American Illustrators. He also won all the professional art honors the World Science Fiction Convention offers, and multiple Chesley Awards from the Association of Science Fiction and Fantasy Artists.
Christensen appeared in an episode of ABC's show Extreme Makeover: Home Edition in 2005. He created a picture featuring a member of the family as a fairy. The design team filmed a segment at his studio. The Greenwich Workshop donated a framed Court of the Faeries that Christensen presented to the family for the room as well.
Christensen has published more than three books, with many of his works appearing in many more. His first book, A Journey of the Imagination: The Art of James Christensen, was printed in 1994 to great acclaim. His second, Voyage of the Basset (October 1996), contains a frame story for a great deal of original work. His third book, Rhymes & Reasons, was published in May 1997. Christensen also illustrated A Shakespeare Sketchbook (May 2001) with text by Renwick St. James.
While not employed in all his paintings, his trademarks were flying or floating fish, often on a leash.
Christensen was married with five children including two notable artist daughters, Cassandra Christensen Barney and Emily Christensen McPhie.
He was a member of The Church of Jesus Christ of Latter-day Saints. Christensen co-chaired the Mormon Arts Foundation with his wife Carole. Among other callings in the LDS Church he served for a time as a bishop of a ward.
He resided in Orem, Utah in a house he designed filled with secret passages and sculptures inspired by his paintings.
Christensen's book Voyage of the Basset was the source of controversy in 2006 when a resident of Bountiful, Utah, demanded that the book be removed from circulation from the young adult section at the Davis County Library in nearby Farmington, Utah. The book features fantasy artwork such as depictions of trolls, dragons and ogres. Two images of mermaids and one of a sphinx-like creature feature partially or fully exposed breasts.
Though the images are not sexual in nature, and as drawn, the breasts feature no nipples, Rod Jeppsen of the Citizens for Decency group said: "What we normally don't consider pornography, a child may get sexually aroused by... The question to me is not whether the book has a good story line, but does it sexually stimulate young boys?" The Davis County Library Board voted to keep the book in circulation in the young adult section on August 22, 2006.
Christensen died January 8, 2017, of cancer.