[6][7] After his PhD, Seward joined the Lawrence Livermore National Laboratory to work on linear accelerator experiments.

[8][9][10] Seward then participated in the 1962 atmospheric nuclear test series in the Pacific, Operation Dominic, and launched Nike-Apache sounding rockets from Johnston Island and Kauai to measure the radiation flux from the Starfish Prime high altitude nuclear explosion.

[7] Seward led a research program to study extrasolar X-ray sources using sounding rockets provided by the Sandia National Laboratories, and pioneered the use of sounding rockets launched from flotation rafts in the ocean in order to study the South Atlantic Anomaly.

[19] Seward formally retired in 2005 but as of 2024 continued to carry out active research on X-ray supernova remnants.

[20] In 2024 he was awarded the biennial George Van Biesbroeck Prize, "For the establishment and implementation of the first open, peer-reviewed Guest Observer program for a Principal-Investigator-led NASA space-based observatory".