He was a founding board member and investor in GRAIL, Inc. in Menlo Park, California, and served for a period as the company's interim CEO (2017) and chair (2017-2018).

[4] At IDEC, Rastetter was a co-inventor of and developed Rituxan, the first monoclonal antibody (MAB) to be approved by the U.S. FDA for cancer therapy.

[4][16] Rastetter was also a co-founder, interim CEO and chairman of Receptos, Inc. a biopharmaceutical company which was bought by Celgene Corporation in 2015.

[4][17][18] Rastetter has been described as an "omnipotent biotech force"[13] by Alnylam Pharmaceuticals CEO John Maraganore and as an "industry legend" who has deep biotechnology expertise.

[3] William Rastetter later married Marisa Gard, an attorney and triathlete[24] who has been featured on the cover of USA Triathlon Life.

Marisa Gard Rastetter is an attorney at the nonprofit San Diego Volunteer Lawyer Program, and runs legal clinics for low-income people, dealing with legal matters as diverse as domestic violence, guardianship, and issues relating to HIV and AIDS.

[4] Rastetter joined IDEC Pharmaceuticals, which was founded in 1985,[28] as of December 1986, shortly after funding of the Series A venture round.

– Rastetter, 2006[32]In 2003, Idec Pharmaceuticals merged with Biogen to form the third largest biotech firm in the United States.

[12] Rastetter serves as an adviser to Illumina Ventures, which focuses its investments in start-up companies driven by genomics and next generation sequencing technologies.

[44][45][46] Rastetter served as chair (2017-2018) and as interim CEO (2017) during a leadership transition at the company which resulted in the hiring of Jennifer Cook.

[7][4][6][13][8] GRAIL's CEO was Hans Bishop at the time of its acquisition by Illumina in 2020 which was disputed by the FTC and European authorities.

Rastetter has a lifelong interest in photography, which began at age 11, when his mother gave him a Kodak box camera.

[3] He experiments with techniques to overlay images[19] and to take extremely long exposures of up to 12 minutes using medium and large format cameras such as the Hasselblad H6D-100c.