Professor Walker subsequently held postdoctoral appointments at the University of London and the Jet Propulsion Laboratory. In 1986 Prof. Walker became a staff scientist in the Concurrent Computation Project at the California Institute of Technology, and in August 1988 was appointed to the University of South Carolina mathematics faculty as an associate professor. In September 1990 Prof. Walker joined the Mathematical Sciences Section of Oak Ridge National Laboratory, where he headed the mathematics group, and became a senior research staff member in 1995. Since December 1995, he has been Professor of High Performance Computing in the Department of Computer Science at the University of Wales Cardiffi where he also serves as Director of the Welsh e-Science Centre.
Professor Walker's research interests focus on software, algorithms, and environments for computational science on high performance computers. He has been closely involved in the development of the ScaLAPACK parallel software library, and the MPI message passing standard. He has also contributed to the design of a parallel version of the Community Climate Model, and has published a number of papers on the parallel implementation of particle-in-cell algorithms for plasma simulations. He has also been involved in the benchmarking of science and engineering applications codes on parallel computers.
Professor Walker has published over 70 papers in the area of parallel computing and has co-authored three books on the subject. In 1992 he founded a series of conferences on high performance computing under the auspices of the Gordon Research Conference organization. He has organized a number of other conferences and workshops in the same area.