Santos was born in Valladolid, Spain.[5] He earned a licenciate in mathematics from the University of Cantabria in 1991, and a master's degree in pure mathematics from Joseph Fourier University in Grenoble, France in the same year.[5] He returned to Cantabria for his doctorate, which he finished in 1995, with a thesis on the combinatorial geometry of algebraic curves and Delaunay triangulations supervised by Tomás Recio.[6] He also has a second licenciate, in physics, from Cantabria in 1996. After postdoctoral studies at the University of Oxford he returned to Cantabria as a faculty member in 1997, and was promoted to full professor in 2008. From 2009 to 2013 he has been vice-dean of the Faculty of Sciences at Cantabria.[5]