He is a professor of hydrology in the School of Environment and Sustainability and the associate director of the Global Institute for Water Security at the University of Saskatchewan.

Following this, he published his thesis The age, origin and pathway of subsurface stormflow in a steep humid headwater catchment in 1989 at the University of Canterbury.

[1][2] Upon completing his PhD, McDonnell was a research hydrologist at the NASA Marshall Space Flight Center from 1989 to 1990.

[6] There he accepted two appointments; one as a professor of hydrology in the School of Environment and Sustainability and another as the associate director of the Global Institute for Water Security.

During the 2013–14 academic year, McDonnell was elected to serve as the president of the Hydrology Section of the American Geophysical Union.